SEA-CUCUMBERS. 215 



glands of the starfish, and their title seems for the most 

 part incorrect. Also connected with the cloaca are little 

 processes — called Cuvierian organs — which the irritated Holo- 

 thurian ejects. 



The water-vascular system may consist of a ring around the 

 mouth communicating (a) with the tentacles, ifi) with five 

 radial vessels, one for each ambulacra! area, (c) with a 

 " Polian vesicle " or more than one of these reservoirs 

 pendent in the body-cavity, (d) with a " stone canal " also 

 hanging in the body-cavity, or opening on the skin. The 

 radial vessels may have, ampullae and tube-feet as in sea- 

 urchins. But there are many divergences, especially in the 

 reduction of the tube-feet areas. Instead of tube-feet, or 

 along with them, there are often conical processes or papillae 

 without terminal discs. These are especially common on 

 the dorsal surface. The blood-vascular system is not very 

 definite, and seems to consist mainly of spaces in the 

 connective tissue. 



The sexes are usually separate. The reproductive organs 

 are branched tubes which open within or just outside the 

 circle of tentacles. They and other internal organs of Holo- 

 thurians are often very brightly coloured. The larva is, 

 in most cases, what we shall afterwards describe as an 

 Aurkularia, and the development is, as in other Echinoderms, 

 very indirect. Sometimes, however, the larval stage is 

 skipped, as in Cucumaria crocea, which carries its young 

 among its dorsal tube-feet, or in Psolus ephippiger, which 

 has a dorsal brood-pouch strengthened by special plates. 



The calcareous plates of Holothurians are found as far 

 back as Carboniferous strata. Trepang or Beche-de-mer is a 

 Chinese delicacy consisting of dried Holothurians. 



Classification. 



1. Elasipoda : primitive deep-sea forms, bilaterally symmetrical, with 



tube-feet on the ventral surface only, and with 



papillse on the back. 

 The stone-canal often opens externally by a pore. 

 There are no respiratory trees. 

 e.g., Kolga, Elpidia. 



2. Pedata : with well-developed tube-feet and papillse. 



e.g., Holothuria, Cucumaria, Psolus. 



