HOLOTHURIANS. 



95 



HOLOTHURIANS. 



SyWI/pta. {Synapta tenuis Ayres.) 



This is one of the most curious of the Holothuriaiis, and easily 

 observed on account of its transparency, which allows us to see its 

 internal structure. It has a 

 long cylindrical body (Fig. rur-iat 



124) along the length of 

 which run the five rows of 

 ambulacra, which are in this 

 instance closed tubes with- 

 out any projecting suckers 

 or locomotive organs of any 

 kind attached to them, so 

 that the name is retained 

 only on account of their cor- 

 respondence in position, and 

 not from any similarity of 

 fdnction to the ambulacra in 

 Star-fishes and Sea-urchins. 

 But though the ambulacra 

 in Synapta are in fact mere 

 water-tubes like the vertical 

 tubes in the Ctenophorse, by 

 means of which the water, 

 first filtered through the 

 madreporic body, circulates 

 along the skin, they are as 

 organs perfectly homologous 

 with the ambulacra in all other Echinoderms. The mouth has 

 a circular tube aroimd the aperture, and a wreath of branching 

 tentacles encircling it. The habits of these animals are singular. 

 They live in very coarse mud^ but they surround themselves with 

 a thin envelope of finer sand, which they form by selecting the 



Vig. 124. Synapta, natoral size. 



