ICHAP. in.] OF MOVEMENTS IN THE ANIMAL KINGDOM. 349 



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

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

 epithelial lining. This tissue 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 



s tegument a muscular layer constituted by fibres confusedly 



interlaced (Fig. 54). 



FIG. 54. 



Section of Hirudo. c, cuticular layer; m, muscular layer ; r, lateral line with the excretory 

 organ ; p, p t median lines, superior and inferior ; g', oblique fibres ; v, intestine ; d, 

 dorsal vessel ; I, lateral vessel ; s, vesicle of the excretory organ ; n, ventral ganglionary 

 chain. 



Already in the echinoderms the case is different. Here the 

 contractile apparatus always forms a subtegumentary tube : but 

 the fibres which compose it are regularly grouped. 2 



In respect to the muscular apparatus, the mollusks 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 



I ! Gegenbaur, Manuel d 1 Anatomic Comparee, p. 118 



2 Ibid., p. 169, 170. 



