Neurons are called excitable cells because these membranes are in a polarised state. Different types of ion channels are present in the neural membrane, i.e., are selectively permeable to different ions.

When a neuron is suitably stimulated an electric disturbance is generated, which swiftly travels along its plasma membrane.

Arrival of the disturbance at the neuron's ending or output zone, triggers events that may cause stimulation of adjacent neuron. These are excitable cells due to differential concentration gradient of ions (specially natural $\mathrm{K}^{+}$) across the membrane.

The special features of membrane of neuron are

(i) Excitability Neurons are able to percieve stimulus and enter a state activity caused by change in electrial potential difference across its covering membrane.

(ii) Conductivity Change in potential difference, spreads through the membrane to the whole neuron.

(iii) Connectivity Neurons are connected to several others, receiving and transmitting impulses to various directions.

(iv) Response The processed message may elicit a response in the form of a sensation.

Earthworms are called 'friends of farmer' because they make burrows in the soil and make it porous which helps plants in respiration and penetration within the soil. The worm casting is rich in urea and ammonia which improves fertility of soil. Thus, they are of much importance to mankind and used as vermicompart.

The body of earthworm can be distinguished into dorsal and ventral sides due to the presence of certain peculiar feature in it like.

(i) The dorsal surface is darker then ventral surface because it is marked by a dark median mid dorsal line along the longitudinal axis of body. This is due to dorsal blood vessel, seen through integument.

(ii) The ventral surface genital openings (pores), both male and female.

(iii) Genital papilla is located on ventral surface and help in copulation.

Among the given statements (b) and (d) are correct statement, correct statement for (a) would be in earthworm there are a pair of male genital pore lying on the ventro-lateral side of the 18 th segment. Male reproductive fluid containing sperms is discharged by these pores. However, a single female genital pore is present in the mid ventral line of 14th segment. Correct statement for (c) Muscular layer in earthworm is consist of an outer layer of circular muscles and the inner layer of longitudinal muscles.

Nephridia are the main excretory organs in earthworms. They are associated with excretory and osmoregulatory functions. The nephridia occur in all segments of earthworm except the first two segments. The three types of nephridia are found in earthworm according to their location.

They are distinguished on the basis of being enteronephric (nitrogen waste expelled input) and exonephric (nitrogen waste discharged outside directly).

Septal Nephridia These are present on both sides of inter segmental septa of the segment starting from 15th to the last that open into intestine. They are enteronephric.

Integumenatry Nephridia These are attached to lining of the body wall of segment 3 to the last that open on the body surface. They are exonephric.

Pharyngeal Nephridia These are present as a three paired tufts in the 4th, 5th and 6th segments. They are also enteronephric.