350 ECHINODERMS OF THE BRITISH ISLES 



it would run a great risk of breaking its neck with any movement 

 of the water and also would evidently be unable to right itself 

 again if it happened to turn over. 



Known with certainty only from off S.W. Iceland, 1515- 

 1640 m. (" Ingolf ") ; but, as stated under the preceding species, 

 it is very probable that the " Porcupine " specimen from the 

 Rockall was really this species. 



V. Class. SEA-CUCUMBERS or HOLOTHURIANS 

 (Holothurioidea) 



Free-living echinoderms, usually with an elongated, worm- or 

 sausage-shaped body, without free arms, but with a circle of 

 tentacles around the mouth, which latter is usually at the 

 anterior end of the body. The oesophagus surrounded by 

 a calcareous ring. Alimentary canal long, mostly looped. 

 Anus usually at the posterior end. Skeleton generally con- 

 sisting of small, microscopical plates imbedded in the skin, 

 sometimes of larger, imbricating plates, w^hich are, hov/ever, never 

 arranged in regular series. Tube-feet never in a furrow. 



The body of the Holothurians is usually elongate, Avorm- or 

 sausage-shaped, more rarely short, radish-shaped ; it is often 

 flattened, showing a distinct ventral and dorsal surface. To the 

 ventral side belong always three radii, to the dorsal side two, 

 the tube-feet being generally most strongly developed on the 

 ventral side, which forms the walking surface. The ventral side 

 is also called the " trivium ", the dorsal side the " bivium ", on 

 account of the relative number of the radii. The skin, which is 

 not cihated, is mostly thick, leathery ; in the group of the Apodous 

 Holothurians, how^ever, generally thin and transparent. There 

 is no continuous skeleton consisting of regularly serially arranged 

 plates, as found in all the other classes of echinoderms, but 

 only small, mostly microscopical calcareous bodies, imbedded in 

 the skin. They may be so numerous and lie so closely crowded 

 together that the skin becomes quite stiff and hard ; in one group, 

 the Psolids, the calcareous bodies have developed into large, 

 imbricating scales. A few forms entirely lack the calcareous 

 bodies. Often the calcareous bodies are of a very beautiful, 

 regular shape, wheels, anchors, etc. ; they are characteristic for 

 every species and are of eminent importance for the classification 

 of the Holothurians. As a rule it is necessary to use a special 

 preparation in order to see them distinctly, clearing up pieces of 



