322 



Mollusea. 



small group of metamorphosed tentacles, whicli are possibly of service 

 in copulation. 



Tlie eggs are either heaped together in masses of mucus or 

 are enclosed in firm capsules. They are of relatively large size ; 

 segmentation is partial, and the embryo often possesses, for a time, 

 a large yolk sac, depending between the arms. There is no 

 metamorphosis, the newly-hatched form resembling the adult. 



The Cephalopoda, which are all marine, are for the most part, 

 predaceous, seizing their prey {e.g., Crustacea) with their arms j the 

 latter are also used for crawling, especially in the Octopods. 

 They may swim by the movement of the fin backwards and 

 forwards; a hurried flight backwards, usually accompanied by a 

 discharge of ink, may be effected by the ejection of the water from 

 the pallial chamber through the funnel. The Decapods are the best 

 swimmers, whilst the Octopods usually crawl better. Many 

 Cephalopods (especially the Decapods, but some Octopods too) 

 often- occur in shoals in the open sea; others a^e more littoral. 

 They are mostly found in warm seas. 



Pig. 268. Diagrammatic figures to illustrate 

 the relations between the Chitons (A) and 

 the Cephalopods (B); profile; / foot (funnel), 

 Ti head, fc mantle cavity, r rim of the mantle, 

 whose upper boundary is indicated by a dotted 

 line, eye. — Orig. 



It is conceivaUe that the cephalopod type was derived from a c h i t o n-like 

 form by enormous development in the height of the dorsal surface, by the 

 deepening of the mantle furrow on the posterior side of this upgrowth, by the 

 great development of the head, and the reduction of the foot. 



Order 1. TetrabrancMata. 



There are numerous arms (tentacles) without suckers. The funnel 

 is a plate rolled upon itself. The eyes have no lens. There are four 

 gills (four auricles, four kidneys) ; no ink sac ; an external shell, 



