Chap, hi.] OF MOVEMENTS IJT THE AKIMAL KINGDOM. 349 



is already a certain differentiation of tissue and a tliin con- 

 tractile layer exists, on the periphery of the body, beneath the 

 epithelial lining. This tissne is already composed of long and 

 fine fibres. 1 



On the contrary, the substance of sponges is formed of cor- 

 puscles, amibiform, amorphous, and contractile. 



In many animals, especially in numbers of worms, in the first 

 period of life, the only locomotory organs are still vibratile 

 cilia : but at the adult age there is developed under the external 

 tegument a muscular layer constituted by fibres confusedly 

 interlaced (Fig. 54). 



J) 



" J -. 



" '<1 c'-^ ■ 



/^*.gfefc^ 



Section of Bvrudo. c, cuticular layer; m, inusaular layer ; r, lateral line with the excretory 

 organ ; p, p, median lines, superior and inferior ; g', oblique fibres ; Vt " intestine ; d, 

 dorsal vessel ; i, lateral vessel ; s, vesicle of the excretory organ ; «, ventral ganglionary 

 chain. 



Already in the echinoderms the case is different. Here the 

 contractile apparr.tus always forms a subtegumentary tube : but 

 the fibres which compose it are regularly grouped. ^ 



In respect to the muscular apparatus, the Hiollusks represent 

 a well-marked link of transition. During the first phases of 

 their development they often move by means of vibratile cilia. 

 At the adult age they are furnished alike with a superficial 



1 Gegentaur, Manuel d'Anatomie Ocrm^wrie, p. 118 

 " lUd., p. 169, 170. 



