MOLLUSC A 195 



The nerve ganglia are usually rendered conspicuous by 

 their bright orange colour. The cerebral ganglia, which lie 

 one on each side of the mouth, probably represent the cerebral 

 and pleural ganglia of other molluscs ; they are united 

 both with the pedal ganglia in the foot and with the olfactory 

 (parieto-splanchnic) situated on the ventral face of the posterior 

 adductor muscle. A pair of auditory vesicles, lined with 

 ciliated ceUs and containing a single otolith, are usually 

 present close to the pedal ganglia, and are innervated by a nerve 

 from the cerebro-pedal commissure, which probably comes from 

 the cerebral ganglia. Tactile papillae or tentacles are common 

 round the edge of the mantle. In some cases the tentacles 

 have been modified and form eyes, which attain a great degree 

 of complexity. In Pecten, Spondylus, etc., these eyes have a 

 remarkable resemblance to the vertebrate type of eye, inasmuch 

 as the optic nerve passes in front of the retina, and the retinal 

 elements are thus turned away from the light. The epithelium 

 in the neighbourhood of the olfactory ganglion is modified to 

 form an organ of smell, by means of which the quality of the 

 water flowing in through the ventral siphon may be tested. 



The LamelUbranchs with few exceptions are dioecious. 

 The generative organs are branched glands usually situated in 

 the foot, though in Mytilus they occur in the mantle. The 

 generative cells are formed in the caecal processes of the gland, 

 and, they leave the body by a right and left simple duct which 

 is continuous with the walls of the gland, and in some cases 

 opens into the duct of the kidney {Spondylus, Lima, and 

 Pecten). 



Division II. GLOSSOPHORA. 



CiiARAOTEElSTlos. — Mollusca with a prostomium more or less 

 developed and a buccal cavity armed with a rasping tongue, 

 the radula, which together with its accessory parts constitutes 

 the odontophore. 

 The Glossophora comprise three classes : 



(i.) Gasteropoda, 

 (ii.) Scaphopoda. 

 (iii.) Cephalopoda. 



