i 7 8 



MANUAL OF ZOOLOGY. 



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. 66, c). They rotate rapidly 

 on their long axis, and have at this stage been described as a 

 distinct genus under the name of Auricularia. 



In the Holothnricz proper, 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 

 Synaptidce there are no ambulacra, but only the central circular 

 canal of the ambulacral system, and the animal moves by 

 means of anchor-shaped spictila, which are scattered in the 



Fig. 66. Annuloida. a Holothuria tubulosa, one of the Sea-cucumbers ; 

 b and c Young stages of the same. 



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 tenta- 

 cula ; 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 secondary 

 vesicles, just as in the Echinus. In the typical forms there 

 are five rows of tube-feet, but these may be scattered over the 

 whole body, or may be restricted to the ventral surface. 

 There is also a " sand-canal," which arises from the circular 

 canal, and is terminated by a madreporiform tubercle ; but 

 this, instead of opening on the exterior, hangs down freely in 

 the perivisceral cavity. The fluid, therefore, with which the 



