56 Invertebrata. 



CLASS IV. Sea Cucumbers (Holothuroidea). 



Singular, elongated or cylindroidal animals, closely 

 allied to sea urchins, but without either spines or hard 

 test, make up this class. They are found among the 

 tangles or in moderately deep water along our shores. 

 The body-wall is muscular, and contains in its surface- 

 layer calcareous spicules, resembling anchors, wheels, 

 &c. The mouth is surrounded by plumose ten- 

 tacles, and, on the surface of the body are rows of 

 tube-feet, more irregularly disposed than in sea 

 urchins. The intestine is moderate in length, and 

 ends in a small aboral sac or cloaca, into which also 

 open (in most species) two very remarkable tree-like 

 organs which lie in the body cavity, and which are 

 adjuncts to the water- vascular system for purposes of 

 Fro breathing. In organisation 



they resemble sea urchins, 

 and often reach very large 

 sizes. In tropical seas some 

 of these Holothurians are 

 inhabited by little parasitic 

 fishes, and even in temper- 

 ate regions small shelled 

 mollusks are found in or 

 I -on their bodies as parasites. 

 I One species of Holothurian, 

 the Trepang, is imported in 

 large quantities from NE. 

 Gwwwwtr^^^^ One of the Australia into China, where it 

 is regarded as a luxury of diet. 

 Recapitulation. Echinoderms are characterised 

 by the presence of highly differentiated tissues, a dis- 



