218 



METAZOAN PHYLA 



the mantle is more or less completely covered with a series of eight 

 overlapping calcareous plates which together make up the shell (Fig. 

 123 A), and the broad flat foot occupies the greater part of the ventral 

 surface. Between the margin of the foot and the edge of the mantle on 

 each side is a row of marginal gill filaments (Fig. 123 B). The nervous 

 system includes four longitudinal nerve cords, united in the chitons to 

 an anterior nerve ring (Fig. 123 C) and in other types to cerebral ganglia. 

 As a whole the system is distinctly wormlike. 



The chitons are all marine and live firmly attached to rocks and other 

 solid objects alongshore between tide marks and also just below the 

 low tide mark. They move with great slowness and adhere so closely 

 to rocks that they are torn loose only by the exercise of considerable 



p/cf/-e 



Lcrf-era/ 

 cor-(pf 





Pecker/ 



MoL/-/"/? 



G///S 



Pos-Z-er/oK 

 C 



Ar^us 



B 



Fig. 123. — Amphineura. A, upper surface of a chiton, Ischnochiton sp. Natural size. 

 B, lower surface of the same chiton. C, diagram of the nervous system of a chiton. (From 

 Cooke, "Cambridge. Natural History,'' after Hubrecht.) D, a primitive, wormlike species, 

 Chaetoderma 7iitidubim Loven. (Also from Cooke, "Cambridge Natural History.") X 2%. 

 Found in the North Atlantic Ocean at considerable depths. Possesses no foot, and has 

 limy spicules in the skin. (C and D by the courtesy of The Macmillan Company.) 



force. The more wormlike and shell-less forms are found on coral polyps 

 and hydroids at depths of 50 fathoms or more. 



259. Gastropoda. — The gastropods possess an elongated flat foot 

 making up the entire ventral surface of the body except for a short por- 

 tion at the anterior end. They also possess a well-developed head and 

 may have a shell, which is often spirally coiled and therefore asymmetri- 

 cal. This class includes a number of very distinct types. 



The type which is most commonly thought of as representative is a 

 snail with a spirally coiled shell. In such an animal the body consists 

 of a head, a more or less distinct neck, a foot, and a visceral mass which 

 is developed into a sort of hump on the dorsal side of the body. The 

 head bears two pairs of fleshy tentacles, a relatively short pair anteriorly 

 which are olfactory in function, and a longer pair posteriorly which bear 

 the eyes at the tip (Fig. 124). These tentacles are hollow and capable of 

 being inverted like the finger of a glove when the tip is pulled down inside. 



