MOLLUSCA. 521 



they reside, it is probable that the skin is besmeared with 

 some unctuous matter. In some instances, indeed, the 

 skin resists being wet, even after the death of the animal 

 has taken place for some time, but previous to becoming 

 dry. 



The Muscles of insects appear to possess the same in- 

 ternal structure as the corresponding organs in the higher 

 classes. They are nearly all simple, and more or less trans- 

 parent and whitish. All those which are concerned in the 

 production of locomotion have their origin and insertion in 

 the skin, apparently without the intervention of tendons. 

 The different members appear to be connected with each 

 other, by the intervention of a more transparent, tough sub- 

 stance, than the skin in other places, to which the name of 

 ligament may be applied.. The action of the muscles will 

 be best understood by a description of the different parts of 

 the body, and the motions which these perform. 



The Head contains the organs of the external senses and 

 the mouth. It is joined to the trunk behind, and has its 

 motions regulated by its mode of connection. Where the 

 head is united to the trunk by a cylinder of ligament, the 

 motion of which it is susceptible is various, limited chiefly 

 in the dorsal direction by the superior margin of the trunk. 

 When the articulation is effected by the immediate contact 

 of the more solid surfaces, the three following modifications 

 of joints present themselves. In the first, there are two or 

 more rounded, smooth tubercles, received into correspond- 

 ing cavities in the trunk. The motion is consequently 

 either backwards or forwards. In the second mode of ar- 

 ticulation, the head is rounded posteriorly, and received 

 into a socket in the thorax. In this manner great liberty 

 of motion is obtained. In many cases, however, it is re- 

 strained by projections of the trunk, which limit it in one 

 direction. In the third, the articulation takes place by the 



