201. An earthworm is transferred from moist soil to a dry surface while air remains available. If water loss from the skin is not prevented, the most likely early consequence is:
ⓐ. reduced skin gas exchange as surface moisture is lost
ⓑ. increased diffusion as the body surface becomes drier
ⓒ. replacement of the skin by specialised respiratory organs
ⓓ. direct delivery of oxygen through nephridial tubules
Correct Answer: reduced skin gas exchange as surface moisture is lost
Explanation: Availability of oxygen in the surrounding air is not sufficient for normal earthworm respiration. The gas must dissolve in moisture on the body surface before it can diffuse across the skin and enter the bloodstream. A dry environment promotes loss of this surface moisture, making the diffusion pathway less effective. The worm does not rapidly develop a different respiratory organ, and nephridia remain excretory and osmoregulatory structures rather than air-delivery tubes. The earliest predicted problem is impaired gas exchange at the external boundary. The primary consequence follows from loss of the named function; broader system-wide failure would require additional changes. This explains the close association of earthworms with moist habitats even though they are terrestrial animals.
202. Equal earthworms are placed at the same temperature and oxygen concentration. Group P remains on moist soil, while Group Q is placed on a drying surface. Over time, Q shows a marked decline in oxygen uptake, whereas P does not. The strongest inference is:
ⓐ. soil particles manufacture oxygen for Group P
ⓑ. specialised lungs become inactive in Group Q
ⓒ. blood vessels function only in wet soil
ⓓ. moist skin supports cutaneous gas exchange
Correct Answer: moist skin supports cutaneous gas exchange
Explanation: Temperature and environmental oxygen are held constant, while the moisture condition differs between the groups. The decline in oxygen uptake occurs only as the surface dries, linking the change to loss of moisture rather than to oxygen scarcity or temperature variation. Earthworms lack specialised lungs, so the observation cannot be explained by lung inactivity. Moisture allows oxygen to dissolve before crossing the body surface, which makes it essential for efficient cutaneous exchange. The experiment supports a requirement for surface moisture under the stated conditions. Only the process directly dependent on the altered component is expected to decline at first. It does not show that every circulatory function stops immediately, only that oxygen entry across the drying surface becomes less effective.
203. Assertion: An earthworm can respire without possessing a specialised respiratory organ.
Reason: Its moist body surface serves as the diffusion boundary through which gases enter or leave the bloodstream.
ⓐ. Assertion is true; Reason is true; explanatory link is valid
ⓑ. Assertion is true; Reason is true; explanatory link is invalid
ⓒ. Assertion is true; Reason is false; explanatory link is invalid
ⓓ. Assertion is false; Reason is true; explanatory link is invalid
Correct Answer: Assertion is true; Reason is true; explanatory link is valid
Explanation: The assertion is true, since earthworm respiration does not depend on a distinct lung, gill, or tracheal organ. The reason is also true and supplies the alternative mechanism. The moist body surface provides a broad boundary across which oxygen and carbon dioxide can diffuse. Oxygen then enters vessels associated with the body wall and is transported internally. The absence of a named respiratory organ must not be confused with absence of respiration. Gas exchange is performed by an existing body surface working together with the circulatory system. Its moist body surface serves as the diffusion boundary through which gases enter or leave the bloodstream; the reason supplies both the relevant condition and the mechanism needed for the assertion. The reason directly explains how respiratory exchange remains possible despite the lack of a specialised organ.
204. A harmless coating keeps an earthworm’s surface moist but is nearly impermeable to respiratory gases. The hearts continue beating and the blood glands remain functional. The most likely outcome is:
ⓐ. improved oxygen entry due to reduced water loss
ⓑ. impaired gas exchange despite continued surface moisture
ⓒ. normal respiration since circulation alone supplies oxygen
ⓓ. increased oxygen uptake through the alimentary canal
Correct Answer: impaired gas exchange despite continued surface moisture
Explanation: Moisture is necessary for cutaneous respiration, but it is not the only requirement. The body surface must also permit gases to cross. The coating preserves water yet creates a physical barrier to oxygen and carbon dioxide, so diffusion falls even though the skin remains wet. Continued heartbeat can distribute gases already present in the blood, but circulation cannot replace entry of oxygen from the environment. Blood glands likewise produce blood components rather than creating oxygen. The case separates moisture maintenance from surface permeability. The hearts continue beating and the blood glands remain functional; preserving this condition confines the immediate inference to the selectively altered feature. Efficient cutaneous respiration requires both a moist boundary and an unobstructed path across that boundary.
205. Link each nephridial type with its characteristic location and route. Each Column II entry is used once.
| Column I | Column II |
|---|
| P. Septal nephridia | W. Three paired tufts in segments \(4\), \(5\), and \(6\) |
| Q. Integumentary nephridia | X. Segment \(15\) to the last; discharge into the intestine |
| R. Pharyngeal nephridia | Y. Segment \(3\) to the last; discharge outside |
ⓐ. P-Y, Q-W, R-X
ⓑ. P-W, Q-X, R-Y
ⓒ. P-X, Q-Y, R-W
ⓓ. P-X, Q-W, R-Y
Correct Answer: P-X, Q-Y, R-W
Explanation: Septal nephridia occur from segment \(15\) to the last and discharge into the intestine, so P matches X. Integumentary nephridia occur from segment \(3\) onward and open to the exterior through nephridiopores, giving Q with Y. Pharyngeal nephridia form three paired tufts located in segments \(4\), \(5\), and \(6\), which fixes R with W. The mapping uses both segmental distribution and outlet relation. Septal and integumentary nephridia extend through many posterior segments but differ in where they discharge, while pharyngeal nephridia are concentrated in a small anterior region. The outlet relation is especially important because location alone cannot distinguish every nephridial type in posterior segments. The outlet relation is especially important because location alone cannot distinguish every nephridial type in posterior segments.
206. The nephridial records are shown below.
| Type | Recorded location | Recorded outlet |
|---|
| P. Septal | Segment \(15\) to the last | Intestine |
| Q. Integumentary | Segment \(3\) to the last | Body exterior |
| R. Pharyngeal | Segments \(4\), \(5\), and \(6\) | Three paired tufts |
| S. Septal | Segments \(4\text{–}6\) | External nephridiopores |
The record requiring correction is:
ⓐ. Record S
ⓑ. Record P
ⓒ. Record Q
ⓓ. Record R
Correct Answer: Record S
Explanation: Record P correctly places septal nephridia from segment \(15\) to the posterior end and states that they discharge into the intestine. Record Q correctly describes integumentary nephridia as extending from segment \(3\) onward and opening outside. Record R identifies the three paired pharyngeal tufts in segments \(4\), \(5\), and \(6\). Record S combines the anterior location of pharyngeal nephridia with the external route associated with integumentary nephridia and labels the result septal. It must be corrected by restoring the posterior septal distribution and intestinal outlet. The entry describing segments \(4\text{–}6\) is paired in the S. Septal record with external nephridiopores; the evidence remains coherent only when both cells are interpreted as parts of one biological pattern. The error involves both position and discharge route rather than a minor naming difference.
207. Damage is confined to segments \(4\text{–}6\) of an earthworm and destroys the paired nephridial tufts associated with the pharyngeal region. Which nephridial type is affected most directly?
ⓐ. septal nephridia extending from segment \(15\) backward
ⓑ. integumentary nephridia distributed through the body wall
ⓒ. pharyngeal nephridia occurring as anterior paired tufts
ⓓ. intestinal caeca arising at segment \(26\)
Correct Answer: pharyngeal nephridia occurring as anterior paired tufts
Explanation: Pharyngeal nephridia occur as paired tufts in segments \(4\), \(5\), and \(6\), so a lesion restricted to this anterior region directly targets that type. Septal nephridia begin much farther posteriorly, from segment \(15\) onward, and integumentary nephridia are widely distributed in the body wall from segment \(3\) onward rather than forming only the named pharyngeal tufts. Intestinal caeca are digestive outgrowths, not nephridia. The diagnosis follows from the combination of segmental position and clustered arrangement. Counting the surviving tufts would add arithmetic without improving the identification. The biologically useful inference is localisation: an anterior three-segment lesion involving paired tufts points to pharyngeal nephridia while leaving the characteristic distribution rules of the other nephridial types unchanged. The restricted segment range distinguishes these tufts from septal and integumentary nephridia, whose distributions begin elsewhere.
208. Read the arrangement described below. Set P is attached to septa from segment \(15\) backward and drains into the intestine. Set Q lies in the body wall from segment \(3\) backward and opens outside. Set R forms three paired anterior tufts. The correct identities are:
ⓐ. P integumentary; Q septal; R pharyngeal
ⓑ. P septal; Q integumentary; R pharyngeal
ⓒ. P pharyngeal; Q integumentary; R septal
ⓓ. P septal; Q pharyngeal; R integumentary
Correct Answer: P septal; Q integumentary; R pharyngeal
Explanation: Set P is identified by attachment to septa, posterior distribution from segment \(15\), and drainage into the intestine; these features define septal nephridia. Set Q lies in the integument or body wall, begins at segment \(3\), and releases waste outside, identifying integumentary nephridia. Set R is restricted to three paired tufts in the anterior segments \(4\text{–}6\), which identifies pharyngeal nephridia. The spatial description supplies three independent clues for each group: tissue position, segmental range, and route or arrangement. Both numerical boundaries and the associated biological feature must agree before the specimen or position is accepted. Reading these clues together prevents confusion between two widely distributed posterior types and the concentrated anterior type.
209. Let \(E\) represent nephridia whose waste route enters the digestive tube and \(X\) represent nephridia whose route opens directly outside. If \(S\) denotes septal nephridia and \(I\) denotes integumentary nephridia, the valid relation is:
ⓐ. \(S \in X\) and \(I \in E\)
ⓑ. \(S \in E\) and \(I \in E\)
ⓒ. \(S \in X\) and \(I \in X\)
ⓓ. \(S \in E\) and \(I \in X\)
Correct Answer: \(S \in E\) and \(I \in X\)
Explanation: Septal nephridia discharge their contents into the intestine, so they belong to the enteronephric set \(E\). Integumentary nephridia open through nephridiopores on the body surface, placing them in the exonephric set \(X\). The valid symbolic mapping is \(S \in E\) and \(I \in X\). The symbols classify outlet routes rather than tissue location alone. Both types participate in excretion and fluid regulation, yet the final path differs: one joins the digestive tube, while the other reaches the exterior directly. The relation captures that route distinction without implying that either nephridial type belongs to a different organ system. Set membership here expresses classification by discharge route, not physical containment of the whole nephridium within an outlet. Membership symbols are appropriate since each named nephridial type is being assigned to a route-defined category. The intestine and body surface are final destinations, whereas the nephridia themselves remain excretory structures in the body.
210. Order the general stages of nephridial processing in an earthworm.
P. Waste-containing material is conveyed through the nephridial tubule.
Q. Excess coelomic fluid enters the nephridial funnel.
R. Material is discharged through an external pore or into the digestive tube.
ⓐ. \(Q \rightarrow P \rightarrow R\)
ⓑ. \(P \rightarrow Q \rightarrow R\)
ⓒ. \(R \rightarrow Q \rightarrow P\)
ⓓ. \(Q \rightarrow R \rightarrow P\)
Correct Answer: \(Q \rightarrow P \rightarrow R\)
Explanation: The nephridial funnel first collects excess fluid from the coelomic region. This material then enters and moves through the nephridial tubule, where the waste-containing fluid is conveyed toward an outlet. The final route may lead to an external pore or to the digestive tube, depending on the type of nephridium involved. The dependency order is collection, tubular passage, and discharge. An outlet cannot receive material before the funnel and tubule have supplied it, while the tubule cannot process or convey fluid that has not first entered. The initial dependency is established when excess coelomic fluid enters the nephridial funnel; the terminal outcome appears when material is discharged through an external pore or into the digestive tube; the order follows biological dependency and not merely the lettering of the stages. The sequence describes the shared functional plan while allowing the terminal route to vary among nephridial types.
211. External nephridiopores are blocked without affecting nephridia that discharge into the intestine. The most direct consequence is:
ⓐ. loss of blood-cell production in segments \(4\text{–}6\)
ⓑ. failure of cutaneous oxygen diffusion across the skin
ⓒ. impaired external discharge by integumentary nephridia
ⓓ. complete stoppage of every nephridial route
Correct Answer: impaired external discharge by integumentary nephridia
Explanation: Integumentary nephridia open to the body exterior through numerous nephridiopores. Blocking those pores prevents their waste-containing fluid from leaving by the normal external route. Septal nephridia discharge into the intestine and are explicitly stated to remain unaffected, so every nephridial pathway does not stop. Blood glands and cutaneous respiration belong to separate systems and are not directly disabled by the outlet obstruction. The prediction follows from matching the blocked structure with the nephridial type that uses it. The change occurs without affecting nephridia that discharge into the intestine; this retained condition keeps the unaffected functions available as a comparison and narrows the inference to the disrupted component. The selective design distinguishes a primary deficit from changes that might arise secondarily. Selective outlet failure can impair one excretory route while an enteronephric route remains available.
212. A tracer is introduced into excess coelomic fluid. In control worms it later appears both at the body surface and in the intestine. In treated worms the external nephridiopores are sealed, and the tracer appears only in the intestine. The strongest inference is:
ⓐ. all nephridia normally discharge only through the body surface
ⓑ. nephridia can discharge outside or into the digestive tube
ⓒ. the intestine produces the coelomic tracer independently
ⓓ. nephridiopores are required for entry into septal nephridia
Correct Answer: nephridia can discharge outside or into the digestive tube
Explanation: In the control, tracer reaches two destinations, showing that more than one outlet route is operating. Sealing external nephridiopores removes the surface appearance but leaves intestinal appearance intact. The treatment distinguishes an exonephric route, used by integumentary nephridia, from an enteronephric route, used by septal nephridia. The experiment does not establish that every nephridium uses both outlets or that the intestine manufactures the tracer. It also does not show that external pores are needed for entry into the funnel. The treated worms retain the intestinal outlet while only the external nephridiopores are sealed; this selective change distinguishes enteronephric discharge from the blocked exonephric route. Surface disappearance of the tracer after pore sealing identifies the external route, while continued intestinal appearance demonstrates that the internal route remains functional. The observations support a limited conclusion that earthworm nephridia can convey collected material to different final destinations according to nephridial type.
213. Assertion: Earthworm nephridia contribute to osmoregulation as well as excretion.
Reason: They help regulate the volume and composition of body fluids while conveying waste-containing fluid toward an outlet.
ⓐ. Assertion is true; Reason is true; explanatory link is valid
ⓑ. Assertion is true; Reason is true; explanatory link is invalid
ⓒ. Assertion is true; Reason is false; explanatory link is invalid
ⓓ. Assertion is false; Reason is true; explanatory link is invalid
Correct Answer: Assertion is true; Reason is true; explanatory link is valid
Explanation: The assertion is true, as nephridia do more than remove metabolic wastes. They also influence how much fluid and which dissolved materials remain within the body, contributing to regulation of internal fluid conditions. The reason states this regulatory function and connects it with the nephridial pathway that collects excess coelomic fluid and conveys material toward an outlet. Excretion refers to removal of wastes, while osmoregulation concerns maintenance of body-fluid volume and composition. These roles overlap within the same tubular system but are not identical terms. They help regulate the volume and composition of body fluids while conveying waste-containing fluid toward an outlet; this is an explanatory relation, not simply two correct statements placed together. The reason directly explains why nephridia are assigned both excretory and osmoregulatory significance.
214. Review the statements given below about the earthworm nervous system.
I. Segmental ganglia occur on a paired ventral nerve cord.
II. In segments \(3\) and \(4\), branches pass around the pharynx.
III. These branches connect with dorsal cerebral ganglia to form a nerve ring.
IV. The main longitudinal nerve cord lies along the dorsal midline above the gut.
ⓐ. I is true; II is false; III is false; IV is true
ⓑ. I is false; II is true; III is true; IV is false
ⓒ. I is true; II is true; III is false; IV is true
ⓓ. I is true; II is true; III is true; IV is false
Correct Answer: I is true; II is true; III is true; IV is false
Explanation: The earthworm nervous system includes a paired ventral nerve cord bearing segmental ganglia, so statement I is valid. In the anterior region, the cord branches in segments \(3\) and \(4\) and passes around the pharynx, supporting statement II. These branches connect with the dorsal cerebral ganglia and form a nerve ring around the pharyngeal region, making statement III valid. Statement IV reverses the orientation of the main nerve cord, which is ventral rather than dorsal. The dorsal position belongs to the cerebral ganglia in the ring relation, not to the longitudinal cord. The valid set preserves both the body-axis orientation and the anterior neural connection. This ventral placement supports rapid coordination of repeated segmental units while the cerebral ganglia remain anterior and dorsal to the pharynx.
215. Examine the arrangement described below. A paired longitudinal cord lies below the alimentary canal and bears repeated ganglia. Near segments \(3\) and \(4\), branches rise on both sides of the pharynx and join a dorsal pair of cerebral ganglia. The complete anterior arrangement forms:
ⓐ. a dorsal blood vessel supplying the pharynx
ⓑ. a nephridial ring discharging into the gut
ⓒ. a circular-muscle band surrounding the oesophagus
ⓓ. a nerve ring linking the ventral cord and brain
Correct Answer: a nerve ring linking the ventral cord and brain
Explanation: The paired longitudinal structure below the alimentary canal is the ventral nerve cord, and the repeated enlargements are segmental ganglia. In segments \(3\) and \(4\), branches from this cord pass around the pharynx and connect with the cerebral ganglia located dorsally. Together, these anterior connections form a nerve ring. The arrangement integrates a ventral longitudinal pathway with a dorsal anterior control centre. A blood vessel, nephridial tube, or muscle layer would not show the stated combination of segmental ganglia and cerebral connections. The figure description establishes the identity through orientation, branching path, and the structures joined. The circumpharyngeal connectives complete the link between the dorsal cerebral ganglia and the ventral longitudinal pathway.
216. The ventral nerve cord is severed immediately behind the pharyngeal nerve ring, while the cerebral ganglia and anterior receptors remain intact. The most likely outcome is:
ⓐ. posterior segments respond normally through the cerebral ganglia alone
ⓑ. the nerve cord shifts to the dorsal side and restores coordination
ⓒ. anterior sensing may persist while posterior coordination becomes impaired
ⓓ. every sensory receptor is immediately converted into a motor ganglion
Correct Answer: anterior sensing may persist while posterior coordination becomes impaired
Explanation: The cerebral ganglia and anterior receptors remain intact, so the worm may still detect certain stimuli near the head. The severed ventral nerve cord, however, can no longer carry normal coordinating signals between the anterior nervous centre and regions behind the cut. Segmental ganglia posterior to the injury may retain limited local activity, but their responses will not be integrated normally with the cerebral ganglia or anterior sensory input. The nervous system does not restore continuity by shifting to the dorsal side, since the main longitudinal cord has a fixed ventral position. The predicted pattern separates stimulus detection from whole-body coordination: an intact receptor may receive information, yet effective transmission to distant posterior segments requires continuity of the ventral nerve cord.
217. The arrangement below is described in words. Structure P lies below the alimentary canal and bears repeated ganglia. Near the pharynx, paired branches rise around it and meet Structure Q above the canal. P and Q are, respectively:
ⓐ. ventral nerve cord and cerebral ganglia
ⓑ. dorsal blood vessel and pharyngeal glands
ⓒ. longitudinal muscle and coelomic epithelium
ⓓ. septal nephridia and intestinal caeca
Correct Answer: ventral nerve cord and cerebral ganglia
Explanation: A paired longitudinal structure positioned below the alimentary canal and carrying segmental ganglia is the ventral nerve cord. In the anterior region, branches from this cord pass around the pharynx and connect with cerebral ganglia located dorsally. The resulting connection forms the nerve ring. The identification depends on three spatial clues: the main cord is ventral, it bears repeated ganglia, and its anterior branches join a dorsal neural centre. A dorsal blood vessel would belong to circulation and would not possess segmental ganglia. Muscle layers and nephridia also lack the described neural connections. The supplied features act together, preventing a nearby structure with only one shared property from fitting equally well. The complete arrangement links the ventral cord with the cerebral ganglia through circumpharyngeal branches.
218. Ganglia in several middle segments of an earthworm are selectively inactivated. Sensory input still reaches the anterior nervous region, but the affected segments no longer produce well-coordinated local responses. The strongest inference is:
ⓐ. cerebral ganglia perform every segmental response without the nerve cord
ⓑ. receptor cells are responsible for all muscular coordination
ⓒ. the dorsal blood vessel replaces inactive neural ganglia
ⓓ. segmental ganglia participate in coordinating regional responses
Correct Answer: segmental ganglia participate in coordinating regional responses
Explanation: The treatment selectively removes ganglionic activity in particular segments while preserving anterior sensory input. The loss of coordinated responses is linked to the missing local neural centres rather than to an inability to detect the stimulus. Segmental ganglia receive and organise information associated with their body regions and connect those regions with the longitudinal nerve cord. The experiment does not show that each ganglion acts independently of the cerebral ganglia; normal behaviour depends on communication across the system. It does show that the ventral cord cannot support ordinary local coordination merely as an uninterrupted cable when its ganglia are inactive. Sensory input still reaches the anterior nervous region; this retained condition makes a system-wide explanation unnecessary and supports a component-specific inference. The observation assigns the segmental ganglia an integrating role between incoming information and regional responses.
219. Assess the following statements about sensory reception in an earthworm.
I. Eyes are absent.
II. Light-sensitive receptor cells permit detection of light.
III. Anterior chemoreceptors contribute to taste.
IV. Sensitivity to ground vibrations requires a specialised external ear.
ⓐ. I and IV only
ⓑ. I, II and III only
ⓒ. II, III and IV only
ⓓ. I, II, III and IV
Correct Answer: I, II and III only
Explanation: Earthworms do not possess eyes, making statement I valid. Absence of an eye does not mean complete inability to detect light. Light-sensitive receptor cells in the body surface can respond to illumination, supporting statement II. Chemoreceptors are especially important in the anterior region and contribute to detecting chemical qualities associated with taste, so statement III is also valid. Earthworms can respond to vibrations transmitted through the ground without possessing an external ear, making statement IV incorrect. Different environmental changes are detected by specialised receptor cells distributed in suitable body regions. The valid combination distinguishes a complex sensory organ from the receptor cells that can still provide sensitivity when that organ is absent.
220. Pair each environmental stimulus with the receptor or response relation most directly involved. Each Column II entry is used once.
| Column I | Column II |
|---|
| P. Change in illumination | W. Touch-sensitive receptor cells |
| Q. Contact with an obstacle | X. Anterior chemoreceptors |
| R. Chemical quality of food | Y. Sensitivity to vibrations transmitted through soil |
| S. Repeated disturbance of the ground | Z. Light-sensitive receptor cells |
ⓐ. P-W, Q-Z, R-Y, S-X
ⓑ. P-X, Q-Y, R-Z, S-W
ⓒ. P-Y, Q-X, R-W, S-Z
ⓓ. P-Z, Q-W, R-X, S-Y
Correct Answer: P-Z, Q-W, R-X, S-Y
Explanation: A change in illumination is detected through light-sensitive receptor cells, so P matches Z. Physical contact with an obstacle activates touch-sensitive receptors, giving Q with W. Chemical information associated with food is detected mainly by anterior chemoreceptors, which fixes R with X. Disturbances travelling through the soil can be perceived as ground vibrations, so S matches Y. The four relations represent different stimulus categories rather than different names for one general response. Light, direct contact, chemicals, and vibration provide distinct forms of environmental information. Correct sensory coordination begins with the receptor system suited to the physical or chemical nature of the stimulus. Each receptor must be matched to the form of environmental energy it can detect.
