MOLLUSCA 245 



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 MoUusca (mollis, soft) embraces about 

 65,000 living species among which there are very great differ- 

 ences 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 frequenters of 

 moist places. Some are well protected by external armor and 

 others are perfectly naked. The typical adult moUusk is 

 clearly marked off from both the radiate animals such as echin- 

 oderms and the segmented animals such as the arthropods and 

 the AnneHda, but some of the simpler types of moUusks, 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. Organ of locomotion a muscular thickening of the body, 

 called the foot; variously modified. 



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

 mantle, usually present, enclosing a space, external to the body, 

 known as the respiratory chamber. 



4. Often a calcareous shell, secreted by the mantle usually 

 becoming either spiral or separated into a right and left valve. 



5. Central nervous system usually consisting 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 (Acephals) a tooth-bearing 

 ribbon, the odontophore, present in the mouth. 



286. General Survey. — The more commonly known forms 

 are easily recognizable by the hard calcareous shell which 

 protects the soft unsegmented body within. The shell may be 



