XI 



PHYLUM MOLLUSCA 



307 



auricles or branchio-cardiac vessels conveying the blood 

 from the ctenidia to the ventricle. 



The nervous system is highly developed, and its principal 

 central parts, representing the 

 cerebral, pedal and visceral 

 ganglia of other Molluscs, 

 with their commissures and 

 connectives, form a ring round 

 the gullet. There are a pair 

 of large eyes situated on the 

 head. In the Cuttle-fishes 

 and other Dibranchiata these 

 have a highly complicated 

 structure, and contain parts 

 analogous to all the principal 

 parts of the eye of a Fish or 

 other Vertebrate ; in Nautilus 

 the eye is of much simpler 

 structure. There is a pair of 

 octocysts, and sensory pro- 

 cesses or depressions sup- 

 posed to be olfactory are also 

 present. Osphradia occur only 

 in Nautilus. 



There are either two (Di- 

 branchiata) or four (Tetra- 

 branchiata) nephridia, which 

 are in the form of sacs opening 

 into the mantle cavity, and 



in the Dibranchiata communicating with the pericardium. 

 Through each of these runs one of the principal veins, 

 round which the secreting tissue of the nephridium is 

 aggregated. 



X 2 



i.d. 



FIG. 181. Sepia Officinalis, enteric 

 canal, a. anus ; b. d. one of the 

 bile ducts ; b. m. buccal mass ; c. 

 caecum ; i, ink-sac ; z. d. ink-duct ; 

 j. jaws ; /. /. liver lobes ; oe. oeso- 

 phagus ; /. pancreatic append- 

 ages ; r. rectum ; .s. g. salivary 

 glands ; st. stomach. (From the 

 Cambridge Natural History.) 



