HOLOTHURIANS 71 



with the exterior by means of a Water-tube opening upon the 

 surface of the body. The position and number of the tube-feet 

 vary very considerably in different species ; thus, in some five 

 regular zones of tube-feet run from mouth to anus ; in others 

 those on the dorsal surface are modified ; in others again the tube- 

 feet may be scattered over the entire surface of the body ; while in 

 the worm-like Synapta and its allies they are entirely absent, 

 the oral ring and tentacles connected with it being the sole repre- 

 sentatives of the water-vascular system. The Holothurians move 

 chiefly by the vigorous extension and contraction of their bodies, 

 which are continually changing their shape by the action of strong 

 muscles ; and sometimes so forcible are the contractions that the 

 whole of the viscera are ruptured and discharged through the 

 cloaca, the animal living for a time without them, until it can 

 make good the loss by growing a new set. 



Some of the Holothurians grow to a relatively considerable 

 size, their round, sausage-shaped bodies measuring over a foot 

 in length, and being capable of very considerable further extension 

 in the process of locomotion. Round our coasts they live among 

 seaweeds, or in sand and mud, with the body concealed and the 

 tentacles exposed. Certain large tropical forms, which abound 

 on coral reefs in the Pacific, are called Trepang, or Beche-de- 

 mer, and are used as food, their capture and curing forming a 

 quite important native industry. One of our British species is 

 popularly called the "Cotton Spinner" (Holothuria nigrd), from the 

 cottony filaments which it discharges when irritated or removed 

 from the water. It may attain to I foot in length when fully 

 extended, and have a diameter of upwards of 3 inches. That side 

 of its body which it keeps uppermost when crawling along is of a 

 dark brownish green colour, while the lower side is a fine yellow. 



The second division, or sub-phylum of the Echinoderma, the 

 Pelmatozoa, 1 is of particular interest as representing a group 

 of animals which are nearly extinct ; indeed, out of its five classes 

 only one, and that the most highly specialised, survives to the 

 present day, though the others must have swarmed in the seas 

 of past geological ages. This surviving class, the Crinoidea, 2 

 comprises the beautiful Feather Stars and their allies, the Stalked 

 Crinoids, which are confined to very considerable depths of the 



1 Greek, pelma, a stalk ; zoo n, animal. 2 Greek, krinon, a lily ; eidos, form. 



