io6 



ANIMAL BIOLOGY 



out as they are extended? Is the respiratory opening on the 

 right or left side of the body? On the mantle fold or on the body? 



(Figs. 202-3-4.) How 

 often does the aperture 

 open and close ? 



Place the snail in a 

 FIG. 203. A SLUG. 



moist tumbler. Does 



the whole under surface seem to be used in creeping? Does the 

 creeping surface change shape as the snail creeps ? Do any folds 

 or wrinkles seem to 

 move either toward the 

 front or rear of its 

 body? Is enough mu- 

 cus left to mark the 

 path traveled? The 

 fold moves to the front, 

 adheres, and smooths 

 out as the slug or snail 

 is pulled forward. 



Cephalopods. The 

 highest and best de- 

 veloped mollusks are 

 the Cephalopods, or " head-footed ' mollusks. Surrounding the 

 mouth are eight or ten appendages which serve both as feet and 

 as arms. These appendages have two rows of sucking disks by 

 which the animal attaches itself to the sea bottom, or seizes fish 

 or other prey with a firm grip. The commonest examples are the 



squid, with a long body and ten 

 arms, and the octopus, or devil- 

 fish, with a short body and 

 eight arms. Cephalopods have 

 strong biting mouth parts and 

 complex eyes somewhat resem- 

 bling the eyes of backboned, 

 FIG. 205. -A SQUID. . 



or vertebrate, animals. Ine 



large and staring eyes add to the uncanny, terrifying appearance. 



The sepia or " ink " discharged through the siphon of the squid 

 makes a dark cloud in the water and favors its escape from 



FIG. 204. CIRCULATION AND RESPIRATION 

 IN SNAIL. 



a, mouth; b, b, foot; c . vent; d, d, lung; 7z, heart. 

 Blood vessels are black. (Perrier.) 



