HOLOTHURIANS. 



181 



In the Dendrochirotse the tentacles are tree-like or branching, and th6re are no 

 Cuvierian organs. The pharynx has retractor muscles. Many of the species inhabit 

 northern regions. Pentacta frondosa, the 

 common searcucumber north of Cape Cod, 

 and extending through the Arctic regions 

 to Great Britain, is from six inches to a 

 foot in length, of a tan brown color, and 

 suggests a cucumber by its shape and the 

 corrugations of its skin. The pharynx is 

 muscular; the stomach short, with well- 

 marked transverse folds within ; the intes- 

 tine several times longer than the body, 

 and the respiratory tree with but one main 

 stem. Connected with the ring-canal are 

 two Polian vesicles, nearly two-thirds as 

 long as the body. The 'single ovary is 

 made up of a mass of long tubes, which 

 are larger than the branches of the res- 

 piratory tree, and are tangled up with 

 them. The genus Cucumaria, or Pen- 

 tacta, has many suckers in the ambulap 

 cral area, and some foniis have them also 

 in the interambulacral areas. The body 

 is short and somewhat five-angled, and 

 there are ten tentacles, of which the two 

 belonging to the odd ambulacrum of the 

 bivium are often smaller than the others. 



In Colochirus the feet are ranged in three rows upon the ventral surface, the 

 remaining ambulacra of the back having only papillae. The two central tentacles of 

 the lower surface are smaller than the others. In C. cceruleus, from the Philippines, 

 the papillae of the back are very long. 



In Psolus the ventral surface is clearly separated from the rest, and there are no 

 suckers upon the upper surface. The surface of the body is covered with compara- 

 tively large plates, of which Psolus complanatus has fourteen to sixteen, in a trans- 

 verse row. This species and another occurs in the PhilijaiJines, P. squamatus is found 

 at the Kurile Islands, P. phantopus and P- fahricii in the North Atlantic, and P. 

 antarcticus in the Straits of Magellan. 



The female of Psolics epMppifer has upon the upper or dorsal surface a group of 

 larger tessellated jjlates, each carried, like the head of a mushroom, upon a pedicel 

 imbedded in the skin. The spaces left underneath these tiny vaults are utilized for 

 the protection of the young, which develop directly into sea-cucumbers. The males 

 have the plates of the back similarly arranged, though there is no marsupium. 



In Thyone, Thyonidium, and other related genera, the suckers of the entire body 

 are alike, and seldom show traces of arrangement in rows. Thyonidium has five pairs 

 of large and five of small tentacles. 



TJiyone briareus lives just below low tide from Long Island Sound to Florida, and 

 is very common. In a specimen little more than three inches long the alimentary 

 canal is aboiit seven feet in length, though the oval stomach is less than an inch. The 



Fio. 159. — Pentacta frondosa. 



