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 Ccelenterata, 

 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. 

 In higher forms the 

 epithelial ingrowths 

 become more independent, and in the Medusae 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. 



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 

 band. (From Carpenter, after Selenka.) 



