244 ZOOLOGY 



What are your conclusions as to the symmetry and the normal position of the 

 squid ? Do you find anything from your external examination which would lead 

 you to class it with the clam and the snail? 



DESCRIPTIVE TEXT 



284. The group Mollusca (mollis, soft) embraces from 

 30,000 to 60,000 living species among which there are very great 

 differences, as illustrated by forms as unlike as slugs, snails, 

 oysters, clams, devil-fishes, and squids. With the exception of 

 a few, they are sluggish animals and largely aquatic or fre- 

 quenters of moist places. Some are well protected by external 

 armor and others are perfectly naked. The typical adult 

 mollusk is clearly marked off from both the radiate animals 

 such as echinoderms and the segmented animals such as the 

 arthropods and the Annulata, but some of the simpler types of 

 mollusks, and the larvae of certain of them which undergo a 

 metamorphosis, strongly suggest that they may be related to 

 some of the worms. 



285. General Characters. 



1. Body soft, unsegmented, bilaterally symmetrical and 

 without segmented appendages. 



2. The organ of locomotion is a muscular thickening of the 

 body, called the foot, which is variously modified. 



3. A thickened dorsal fold of the body wall, called the 

 mantle, is usually present. This encloses a space, external to 

 the body, known as the respiratory chamber. 



4. The mantle secretes in many cases a calcareous shell, at 

 first single and symmetrical, but usually becoming either spiral 

 or separated into a right and left valve. 



5. The central nervous system usually consists of three sets 

 of ganglia: (i) the cerebral or "brain," above the mouth, (2) 

 the pedal, in the foot and connected with the cerebral by nerves, 

 and (3) the visceral, also connected with the brain by a pair of 

 nerves (Fig. 38). 



6. Except in the headless forms (Acalephs) a tooth-bearing 

 ribbon, the odontophore, is found in the mouth. 



286. General Survey. The more commonly known forms 

 are easily recognizable by the hard calcareous shell which 



