14 FORM. SKIN. SHELL 



nerves which arise from each of these ganglia, to be distributed 

 to the different organs, are at first simple, but very soon pre- 

 sent in different parts of the body ganglionic swellings ; but 

 these ganglia, however numerous they may be, never form a reg- 

 ular longitudinal chain, extending along the middle line of the 

 abdominal face of the body, as is the case in the articulated ani- 

 mals. 



6. As we have already said, in this great branch of the animal 

 kingdom, there is neither an internal skeleton, analogous to the 

 solid frame of vertebrate animals, nor an external skeleton, sim- 

 ilar to the tegumentary sheath, which envelopes the whole body 

 of articulate animals in a series of rings, and serves the same 

 purposes as the skeleton, properly so called, of the superior ani- 

 mals. The general form of mollusks is extremely variable. 

 Their body is always soft, and in a very small number of them 

 only, there exists internally some solid pieces which are unarticu- 

 lated, and serve rather to protect the viscera than to furnish 

 levers and points of support to the apparatus of locomotion. The 

 muscles are attached directly to the integuments and act but 

 very little beyond their point of insertion ; their motions are slow, 

 and in general ill directed. In a small number of these beings, 

 there are elongated and flexible appendages, designed for locomo- 

 tion (jig. 7, page 23) ; but in most instances the animal can 

 change its place, only by successively contracting different pointa 

 of the inferior surface of its body, and, even when it possesses 

 extremities, they are united in a group at one end of the body, 

 and never arranged in a symmetrical series, as they are in verte- 

 brate and in articulate animals. 



7. The skin of mollusks, always soft and viscid, often forms 

 folds that more or less completely envelope the body, and this 

 disposition has caused the name of mantle to be given to thai 

 portion of integument which ordinarily furnishes these expansions. 

 Frequently this mantle or pallium is almost entirely free, and 

 constitutes two large veils which conceal all the rest of the ani- 

 mal, or rather, these two laminae or membranes unite together so 

 as to form a kind of tube; but at other times, it consists only of 

 a sort of dorsal disk, the edges of whicft alone are free, or sur- 

 round the body more exactly under the form of a sack. 



8. In general this soft skin is protected by a sort of stony 

 cuirass named shell. It is a secretion having some analogy to 



6. What kind of a skeleton have mollusks ? What are the general cha- 

 racters of their body ? How is locomotion effected in mollusks ? 



7. What is the mantle or pallium? 



8. What is meant by shell ? What is meant by naked mollusks ? What 

 are testacea ? 



