. Elements of Comparative Anatomy. FOEM OF BODY OF MOLLUSCA. 319 secoudaiy and variously-diiSerentiated axes—tlie dorso-veutral and the transverse. The body is therefoi'e of the original eudipleural form, which is the dominant one in the Vermes and Artliropoda. These relations are different in the Gastropoda, where the dorsal cup-like shell gradually encloses the greater part of the body, and leaves a small portion only of the surface of the body exposed in addition to the head and foot. So that while in the previous case the shell was adapted to the body, in this case the soft parts of the bo
. Elements of Comparative Anatomy. FOEM OF BODY OF MOLLUSCA. 319 secoudaiy and variously-diiSerentiated axes—tlie dorso-veutral and the transverse. The body is therefoi'e of the original eudipleural form, which is the dominant one in the Vermes and Artliropoda. These relations are different in the Gastropoda, where the dorsal cup-like shell gradually encloses the greater part of the body, and leaves a small portion only of the surface of the body exposed in addition to the head and foot. So that while in the previous case the shell was adapted to the body, in this case the soft parts of the body are adapted to the single shell. The body, therefore, becomes asymmetrical, and the aboral pole no longer carries the anus, which becomes lateral in position in consequence of the flexure of the enteron; this flexure is due to the formation of the shell. All of the many variations from the symmetrical ground-form, which are seen in the Gastropod-body, may be r-egarded as due to this. The primitive similarity in the form of the body, due to the possession of a shell, undergoes great modifications even among the Gastropoda; the Veliger stage is not always developed, and has never yet been observed in the Cephalopoda. But even in this class the form of the body, and the disposition of its viscera, m^ay be seen to be, in all forms, due to the possession of a shell. § 249. The velum has different functions in different divisions. In the LameUibranchiata, where it functions for some time as a locomotor organ, but where it is never independent and soon atrophied, its function is not very great. This may, perhaps, be correlated with the rudimentary character of the future head, and this, again, with the rapid disappearance of the free mode of life in this division (Acephala). Two folds, however, which are given oif laterally from the dorsal surface, become considerably developed and form a mantle; they surround the body, and excrete the shell, which corresponds A ^—^ B with the
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