Chap. III.] MOLLUSCA. 79 



is protected by a hard body, the shell (#), and this 

 shell is derived from a primary shell-sac (/) ; the 

 walls on either side of the middle line of the body are 

 produced into free folds, the pair of which make up 

 the mantle, and on either side of the body there are 

 given off comb-like processes (ctenidia) (i), which 

 are ordinarily known as the gills. Indications of 

 metameric segmentation are rare, and are only 

 obscurely indicated in the majority of the cases where 

 they are to be detected. 



The Mollusca may be primarily divided into those 

 in which the region of the head is reduced or lost, 

 and those in which it takes on more special characters. 

 The former are conveniently known as : 



A. JLipocephala. This division contains only 

 the group *of the JLamellibranchiata or mussels 

 and oysters. In these the primitively single shell is 

 divided into two bilaterally symmetrical halves, and 

 the two divisions of the shell are only different 

 (Oyster : Myodora) in size and character, when one 

 side comes to be that on which the animal ordinarily 

 reposes, or when it ceases to live in an upright 

 position ; the foot may, as in boring forms, be of con- 

 siderable size, or it may be greatly reduced, as in the 

 oyster, which remains for long periods at the same 

 place. 



This shell is brought together by special adductor 

 muscles, of which two pairs are found in many adults, 

 and have been observed in the young of some which 

 (oysters) have only one pair in adult life. The 

 ctenidia, which commence as separate ciliated filaments 

 in two rows on either side, ordinarily undergo a large 

 amount of fusion or concrescence, whereby they are 

 converted into perforated plate-like structures which 

 have, among others, a respiratory function. In some 

 the mantle never extends beyond the limits of the 

 shell, and these are the : 



