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



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 destitute of symmetry, the radiated arrangements of parts seen in 

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

 characteristic of the higher sub-kingdoms. In the more elevated Mollusca, 

 however, which possess not merely sensitive tentacula, but eyes and even or- 

 gans 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 ; evi- 

 dently bearing a pretty close relation 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, as also in the structure of many of the individual 

 organs. It is also manifested in many of the shell-less Gasteropoda, such as 

 the Slug, or the Aplysia (Sea-Hare) ; as will be seen by the accompanying 

 representation of a species of the latter. But this symmetry does not extend 



Fig. 2. 



Aplysia depilans ; a, branchiae or gills. 



to the arrangement of the internal organs ; and appears to be only designed to 

 adapt the body for more convenient locomotion. 



13. As a group, however, the Mollusca are to be characterised rather by 

 the absence, than by the possession, of any definite form ; arid there is a 

 corresponding absence of any regular organs of support, by which such a 

 form could be maintained. The name they have received designates them 

 as soft animals ; and this they are pre-eminently, as every one knows, who 

 has taken a Slug between his fingers. The shell, where it exists, is to be 

 regarded rather in the light of an appendage, designed for the mere protec- 

 tion of the body, and deriving its shape from the latter, than as a skeleton, 

 giving attachment to muscles, and regulating the form of the whole structure. 

 It is in no instance a fixed point for the muscles of locomotion ; and it is only, 

 indeed, where the body is uncovered by a shell, or where a locomotive organ 

 may be projected beyond it, that any active movements can be executed. 

 This locomotive organ, ihefoot, as it is commonly termed is nothing else 

 than a fleshy mass, formed by the increased development of the muscular 

 portion of one part of the general envelope of the body, termed the mantle, 

 in which the visceral mass is loosely included. The mantle is not essen- 

 tially different from the skin of other animals ; but it is usually thicker, pos- 

 sessing a considerable amount of muscular fibre interwoven with it, and its 



