320 MOLLUSCA. 



visceral commissures very frequently anastomose (Cephalopoda, 

 Gastropoda). 



Tactile organs. In addition to the general sensibility of the 

 skin there are tentacles on the head, or in the Acephala on the 

 mantle-edge. The olfactory sense is partly discharged by the ten- 

 tacles, and partly by the osphradia, which are sensory patches of 

 the mantle epithelium near the base of the gills, and are innervated 

 by nerves from the visceral commissure. The eyes have generally a 

 complicated structure; there are usually two on the head, and in 

 rare cases they are found on the mantle-edge (Acephala), and even 

 on the dorsal surface (Chiton, Onchidium). 



Auditory organs, or organs which, from their structure, are 

 supposed to have that function are very generally present. They 

 have the form of otocysts, provided with sense-hairs on their 

 internal walls, and containing one or more otoliths. They are 

 usually paired, and lie either on the cerebral or pedal ganglia. 

 They are, however, said to be always innervated from the former. 



The alimentary canal (absent in two parasitic forms, Entoconclia 

 .and Entocolax) is always provided with a mouth and anus. The 

 mouth is always at the front end of the body, on the head in 

 the Cephalophora, and the anus opens into the mantle-cavity 

 behind, excepting in Gastropods, in which the mantle-cavity is on 

 the front side in consequence of the torsion of the visceral sac. 

 Three parts can be distinguished^ (1) the stomodaeum, consisting 

 of buccal cavity and oesophagus; (2) the mesenteron, consisting of 

 stomach and intestine; and (3) the proctodaeum, which is very 

 .short in most forms, even if it can be distinguished at all. 



The buccal cavity is absent in Acephala (Lamellibranchs) ; but 

 in the other forms, which may be called Odontophora, it forms 

 a chamber with muscular and cartilaginous walls, which receives 

 the salivary ducts, and has on its floor an apparatus called the 

 odontophore. This consists of a toothed chitinous ribbon the 

 radula, a pair of cartilaginous pieces beneath the buccal floor, 

 and a sheath in which the ribbon is formed (for detailed descrip- 

 tion see under Gastropoda, p. 356). There is usually an extensive 

 liver opening into some part of the mesenteron. 



The relations of the perivisceral or body-cavity, in the formation 

 of which the coelom and haemocoele take varying parts, have already 

 been described, and will be dealt with again in detail under the 

 various classes. 



The vascular system forms a completely closed system. Even 



