Sensory organs are well developed and are as follows:
(i) Osphradium:
It is suspended from the roof of the mantle in the left nuchal lobe. It is small 60-70.0 mm long, broad in the middle.
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Its inner end is bluntly round while the outer end is somewhat pointed. U is bipectinate consisting of 22-28 thick and fleshy leaflets, arranged in two rows-on either side of central axis. It is supplied by a nerve from the left pleural ganglion.
In a transverse section, the outermost layer is epithelium followed by a thin basement membrane.
The inner cavity is filled with connective tissue, nerves and blood spaces, three types of epithelial cells viz sensory, ciliated and glandular are found.
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Osphradium serves as an olfactory organ and tests the chemical nature of the incoming water current.
In case the water is foul its entry into the mantle cavity is stopped by the closure of the left nuchal lobe. It may also help in the selection of food material.
(ii) Eyes:
A pair of small black eyes is lodged in stumpy stalks, called ommatophore, one on either side of the head behind the second pair of tentacles.
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In a transverse section an eye appears as a pyriform cup-like invagination of epidermis called the optic vesicle surrounded by a firm sheath of connective tissue.
Cavity of the vesicle is filled up by an oval, structureless hyaline body, the lens. The epithelium of the vesicle is modified posteriorly to form a sensory and pigmented retina. It contains two kinds of large cells
(i) broad visual cells having a brush of hair like processes on their outer ends
(ii) Narrow supporting or packing cells lying in between the visual cells.
Over the lens, retina extends and is modified into transparent cornea or pellucida interna. The epithelium of vesicle also extends above the cornea and is modified into outer cornea or pellucida externa.
Inspite of their elaborate structure eyes of pila are probably not true organs of sight. They repond to changes in the intensity of light and detects quick movements only.
(iii) Statocysts:
They are a pair of small pyriform sacs, lying one on either side of the pedal ganglia attached by a band of connective tissues.
Each statocyst is a hollow capsule surrounded by an outer thick, tough and leathery covering of connective tissue.
It is lined by a single layer of ectodermal cells and supplied by a nerve from the cerebral ganglion.
Cavity of statocyst is filled with fluid containing oval and calcareous particles, the statoconia. These are the organs of equilibrium.
(iv) Tentacles:
Two pairs of tentacles, first pair is called labial palps are found. Behind the first pair, second pair of tentacles is long filamentous, highly contractile whip-like processes.
Tentacles are of the same colour as the snout, and they are tactile in function. A sense of taste is doubtfully attributed to the labial palps.