HOLOTHURIANS. 179 



Order I. — ELASIPODA. 



The Elasmapoda, or Elasipoda, are true deep-water forms, none of which are 

 known to exist at a less depth than fifty-eight fathoms, at which level JElpidia 

 glacialis has been taken in the Arctic Ocean. The same species has been dredged 

 in warmer seas in two thousand six hundred fathoms. The first example of this group 

 was discovered in the Kara Sea over seven years ago, but from the results of the recent 

 deep-sea exploring exijeditions over fifty sjjecies are now known. 



In the Elasmapoda the adult and larval forms agree more closely than in other 

 holothuroids, and they are therefore j)laced by some naturalists low in the scale, while 

 others place them high on account of the distinct bi-lateral symmetry of their bodies, 

 the well-marked distinction of the dorsal from the ventral surface, and the frequent 

 specialization of a cephalic or 'head' portion. The ventral ambulacrse alone are fitted 

 for locomotion. 



Kblga hyalina is a small example of the Elasmapoda, with the oral disc facing the 

 ventral surface, and the anal orifice facing the dorsal surface, thus j)roducing an arrange- 

 ment similar to that which prevails in the star-fishes and echini. The anal orifice has 

 a dorsal collar, bearing sucker-like contractile jjapillsB, communicating with the body- 

 cavity instead of with the water-system. The sand canal (stone canal), instead of 

 hanging free within, opens on the exterior in front of the dorsal collar. This is simply 

 a persistence of the condition of things found in the larvae. Two other genera, Tro- 

 chostoma and Irpa, have the outer end of the sand canal attached to the skin, with a 

 madreporic plate at the point of attachment, but the canal does not communicate with 

 the exterior. In Elpidia the arrangement is similar, but the madreporic plate is rudi- 

 mentary or wanting. Kolga is dicecious, but has no respiratory tree. The two dorsal 

 nerve-trunks furnish an offshoot to each of a pair of large vesicles containing otoliths 

 — a rudimentary organ of hearing. 



Oedek XL — APODA. 



In this order there are no ambulacral feet, and the water-system is therefore re- 

 stricted to the ring around the gullet, the circlet of tentacles around the mouth, and 

 the canal communicating with the madreporic body. The Apoda are again divided 

 into the Apneumonia, which are destitute of a respiratory 

 tree, have no proper cloaca, and are without Cuvierian 

 organs, and the Pneumophora. 



The simplest of the footless holothurians without 

 breathing organs, and, indeed, the simplest of all known 

 holothurians, is JEupyrgus scaber, a species less than half 

 an inch in length, provided with a circle of fifteen un- 

 branched tentacles round the mouth, and covei-ed with soft 

 papillae bearing calcareous plates. The longitudinal muscles 

 are weak and small. It is an Arctic species, and has been 

 taken on the coasts of Labrador, Greenland, and Norway. fig- ist. - ' wheel ' from skin of 



, . Mynotrochus nnkii, 



Myriotrochus rinkii has also been found in shoal water 

 on the coast of Labrador, and has a transparent skin dotted with minute white spots. 

 These spots, when magnified, are seen to be wheel-like calcareous plates. 



