106 INVERTEBRATE ANIMALS. 



the least highly-developed sections of the whole class of the 

 Echinodermata. 



Oedee VII. HoLOTHUEOiDEA. — In this order are comprised 

 the highest of the Echinodermata, all very dififerent in out- 

 ward appearance from any of the forms we have hitherto con- 

 sidered. They are commonly known as sea-cucumbers, or tre- 

 pangs, but they are mostly rare and inconspicuous animals at 

 the best. They are all more or less worm-shaped or snail-like 

 in form, and they are either altogether destitute of calcareous 

 matter in the skin, or with rare exceptions have only scattered 

 grains and spines of this material. As a rule, the skin is 

 simply leathery, and is endowed with wonderful contractility 

 by means of powerful longitudinal and transverse muscles. 

 In consequence of this, they can, in many cases, eject all or 

 almost all their internal organs, and can sometimes divide their 

 bodies into several parts when injured or alarmed. Loco- 

 motion is effected by alternate extension or contraction of their 

 worm-like bodies, by anchor-shaped spicules of lime contained 

 in the skin, or by rows of ambulacral tube-feet, like those of 

 the sea-urchins, protruded through the integument. Some- 

 times the tube-feet are scattered over the whole surface of the 

 body, and sometimes they are altogether absent. There is 

 always a mouth at one extremity of the body, and a distinct 

 vent at the other. The mouth is situated anteriorly, and is 

 surrounded by a circlet of feathery tentacles (Fig. 38), which 



m-^. 



Fig. 8S.— Holotliuroidea. Thyone papulosa (after Forbes). 



are believed to be modified tube-feet. The water-vascular or 

 ambulacral system is sometimes quite rudimentary, but in 

 other cases it much resembles that of the sea-urchins, except 

 that the madreporiform tubercle is not placed on the outside 

 of the bodj', but hangs down freely in the interior of the body. 

 In most of the Holothuroidea there are appended to the ter- 

 mination of the intestinal canal two much-branched tubes, 



