376 INVMBTEBBATE MOBPHOLOGT. 



CHAPTER XII. 



TYPE MOLLUSOA.. 



"While the Annelida are characterized by an elongated 

 form of body, the MoUusca present the opposite condition, 

 being compact, non-metameric organisms, though at the same 

 time primitively bilateral in the arrangement of their organs. 

 Upon the external surface of the body a cuticular secretion 

 is formed in which usually particles of carbonate of lime 

 are deposited, a calcareous shell being thus developed, which 

 encloses more or less perfectly the soft body, assuming, 

 however, very different forms in the various groups. It is 

 essentially a dorsal structure developed in the majority of 

 forms from a depression on the dorsal surface of the body 

 — the shell-gland (Fig. 122, /) — and in some forms may be 

 entirely confined to this area. Usually, however, a circular 

 or bilateral fold of the body, the mantle (c), arises peripheral 

 to the margins of the shell-gland and extends downwards 

 towards the ventral surface, and the growth of the shell may 

 accompany that of the mantle-fold, so that the entire body is 

 enclosed by or may be retracted within the greatly-developed 

 shell. Even in cases, however, in which the shell is but 

 slightly developed the mantle-folds retain their development, 

 forming a marked structural feature of the MoUusca, and en- 

 closing a more or less spacious cavity, the mantle-cavity, in 

 which lie the respiratory organs and into which the intestine 

 and nephridia and reproductive ducts open. 



The body-wall is formed of an external layer of ectoderm, 

 below which a more or less thick layer of muscle-tissue is 

 found whose fibres sometimes show the arrangement in cir- 

 cular and longitudinal layers characteristic of the Annelida, 

 but usually the simplicity of this arrangement is interfered 

 with by a development of connective tissue in which irregu- 

 larly-arranged muscle-bundles lie. Upon the ventral surface 



