38 ON THE PLACE OF MAN IN THE SCALE OF BEING. 



elongation of the body, still seldomer with any indication of that division 

 into segments, which are the chief peculiarities of the Articulata. It is by 

 the absence of these, and of any trace of the Vertebrated structure, that the 

 Mollusca are most readily defined. The variety of form which they present 

 is less surprising, when it is considered, that the bulk of their bodies is almost 

 entirely made up by organs of nutrition ; and the organs of sensation and loco- 

 motion which they possess, are subservient to the supply of these. We find, 

 in the lowest tribes of this group, living beings which are fixed to one spot 

 during all but the earliest period of their lives ; and which scarcely possess 

 within themselves so much power of movement as that enjoyed by the in- 

 dividual Polypes in a compound polypidom ; and yet these exhibit a complex 

 and powerful digestive apparatus, a regular circulation of blood, and an active 

 respiration. We never find, throughout the whole Animal kingdom, that the 

 apparatus of organic life is arranged on any definite plan of its own ; its con- 

 formation is adapted to the type which predominates in the structure of each 

 group, and which is principally manifested in the disposition of the locomotive 

 organs. Thus, the stomach of the Star-fish is circular, and sends a prolongation 

 into each ray ; whilst the digestive cavity of the Articulata is prolonged into a 

 tube. In the Mollusca, there is no such definite type, the apparatus of nutri- 

 tion having the predominance over that of locomotion ; and the form of the 

 body is, therefore, extremely variable. The relative places, even of the most 

 important organs (such as the gills), are found to undergo complete changes, 

 as we pass from one tribe to another ; although their general structure is but 

 little altered. 



23. The lower Mollusca may be characterized as consisting merely of a bag 

 of viscera ; they have not even any prominence for the mouth, nor any organs 

 of special sense, such as would distinguish a head; and they are entirely desti- 

 tute of symmetry, the radiated arrangement of parts seen in the lower tribes 

 being absent, as well as the bi-lateral correspondence which is characteristic 

 of the higher. In the more elevated Mollusca, however, which possess not 

 merely sensitive tentacula, but eyes and even organs of smell and hearing, we 

 find these disposed in a symmetrical manner; so that the head, which is the 

 part concerned peculiarly in animal life, does present a bi-lateral equality of 

 parts, even when the remainder of the body wants it. Further, in the more 

 active among the higher classes, we find this bi-lateral symmetry showing 

 itself in the exterior of the whole body ; evidently bearing a pretty close rela- 

 tion to its degree of locomotive power. It is most evident and complete in 

 the Cephalopoda (Cuttle-fish tribe); many of which are adapted to lead the 

 life of Fishes, and resemble them in the general form of the body, and in 

 the structure of many of the individual organs. It is also manifested in many 



Fig. 2. 



Aplysia depilans ; a, branchiae or gills. 



