GENERAL CHARACTERS OF MOLLUSCA. 39 



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 to the arrangement of the internal organs ; 

 and appears to be only designed to adapt the body for more convenient loco- 

 motion. 



24. As a group, however, the Mollusca are to be characterized rather by 

 the absence than by the possession, of any definite form ; and there is a cor- 

 responding 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 protection 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 in- 

 stance 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 pro- 

 jected beyond it, that any active movements can be executed. This locomotive 

 organ, the foot 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 essentially different from the skin 

 of other animals ; but it is usually thicker, possessing a considerable amount 

 of muscular fibre interwoven with it, and its surface having frequently a glan- 

 dular character. This general muscular envelope is the only locomotive organ 

 possessed by a large proportion of the Mollusca; but its contractile properties 

 are usually greatest at some particular spot, where it is thickened into a sort 

 of disk, by the alternate contraction and extension of which the animal can 

 slowly propel itself; this is well seen, by causing a Snail or Slug to crawl over 

 a piece of glass, so that the under surface of the disk may be seen whilst it 

 is in operation. The general character of their locomotion, however, is well 

 expressed by the term sluggish; and there are scarcely any among the typical 

 Mollusca, whose activity is such as to demand for them any higher appellation. 



25. The general development of their organs of Nutrition, however, is much 

 higher than is met with among the Articulata; and, in proportion to that of 

 the organs of Locomotion, it is much greater than will be elsewhere observed 

 throughout the Animal kingdom. The justice of this statement will be made 

 evident by a slight examination of the adjoined figure, in which the interior 

 structure of the Jlplysia, showing the general character of that of the group, 

 is displayed. The only set of muscles which this animal possesses, is that 

 connected with the mouth, which it is able to push forwards or to draw back ; 

 and which possesses considerable powers of mastication, and is furnished with 

 large salivary glands. The nervous centres (of which more will be said here- 

 after) are seen to be principally disposed around the oesophagus. The whole 

 digestive apparatus is observed to be very complex and highly-developed ; the 

 liver alone occupies a considerable part of the cavity. The heart has distinct 

 muscular walls, and is divided into a separate auricle and ventricle ; and a 

 large respiratory organ is developed for the aeration of the blood. The posi- 

 tion of the gills, which are external to the cavity, but which are concealed in 

 part by a fold of the mantle, and in part by the rudimentary shell, is seen at 

 , Fig. 2. The generative apparatus, also, is highly developed. Yet with 

 all this complex organization, the locomotive power of tfye animal is not much 

 greater than that of the Slug ; no other means being provided for the purpose, 

 than the contractility of the general envelope, which is greatest on the under 

 side of the body. 



26. The blood of the Mollusca is white, and the number of corpuscles in 

 it is small. Their temperature is low, being seldom more than one or two 



