372 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



all but it, the members of which are always fixed 

 when adult, a definite tissue, or collection of cells, 

 becomes specially endowed with a contractile function, 



and forms muscular 

 tissue, and a more 

 or less regularly 

 disposed muscular 

 system. (For the 

 minute structure of 

 muscle see " Klein's 

 Histology," chaps, 

 viii. and ix.) 



In Hydra, among 

 the Coelenterata, 

 the only indications 

 of muscular tissue 

 are the branched 

 prolongations in- 

 wards of certain of 

 the cells of the 

 ectoderm (neuro- 

 muscular cells of 

 Kleinenberg, or, 

 more shortly, Klei- 

 nenberg's cells) ; in 

 it the several cells 

 of the body still re- 

 tain their indepen- 

 dent contractility. 



Fig. 158. Larva of Holothuria tubulosa in 

 its natural position. 



The arrow indicates the axis of rotation, and 



the cilia are seen to be arranged in a sinuous T i i < -T 



band. (From Carpenter, after Selenka.) Ill higher lOrniS the 



epithelial ingrowths 



become more independent, and in the Medusa? they 

 become transversely striated. In these last they form 

 a sheet on the lower face of the disc or umbrella, which 

 in living specimens is repeatedly opening and closing ; 

 they are continued into the tentacles, and when a 

 velum is present they are largely developed in it. 



