ECHINODERMATA : HOLOTHUROIDEA. 



219 



body into an equal number of longitudinal segments or lobes. 

 The mouth is surrounded by a circlet of feathery tentacles, con- 

 taining prolongations from the central ring of the water-vascu- 

 lar system ; and an anus is situated at the opposite extremity 

 of the body. There is a long, convoluted intestine. A special 

 respiratory, or water-vascular, system is often developed, in the 

 form of a system of arborescent tubes, which admit water from 

 the exterior. The larva is vermiform, and has no skeleton. 

 At a certain period of their existence, the young Holothurians 

 are barrel-shaped, with transverse rings of cilia (fig. 109, c). 



Fig. 109. Holothuroidea. a Holothuria tubulosa, one of the Sea-cucumbers 

 b and c Young stages of the same. 



They rotate rapidly on their long axis, and have at this stage 

 been described as a distinct genus under the name of Auri- 

 cularia. 



In the typical Holothurians, locomotion is chiefly effected 

 by means of rows of ambulacral tube-feet, or by alternate ex- 

 tension and contraction of the worm-like body ; but in the 

 Synaptida, in which there are no ambulacra, and only the 

 central circular canal of the ambulacral system is present, the 

 animal moves by means of variously shaped spicula, which are 

 scattered in the integument. When developed, the ambulacral 

 system consists of a " circular canal/' surrounding the mouth, 

 bearing one or more " Polian vesicles," and giving off branches 

 to the tentacula; and of five "radiating canals" which run 

 down the interspaces between the great longitudinal muscles. 

 These radiating canals give off the tube-feet and their second- 



