128 ZOOLOGY. 



treme low-water mark to a depth of fifty fathoms. It is of 

 a tan-brown color, from six inches to nearly a foot in 

 length, and in its form and the corrugations of its tough, 

 leathery skin resembles a cucumber in nearly all respects 

 except color. There are five series of ambulacral feet, each 

 series consisting of two irregular rows. Around the mouth 

 is a circle of ten much-branched tentacles or gills (homolo- 

 gous with the ambulacral feet). 



On laying the body open by making a cut extending from 

 the mouth to the vent, the thick muscular walls of the body 

 may be observed, and the general relations of the viscera to 

 the body-walls, which have nothing of the radiate airrange- 

 ment of parts, so clearly marked in the other Echinoderms, 

 the ambulacra^ tentacles, and longitudinal muscles alone be- 

 ing arranged in a radiate manner.* Unlike other Echino- 

 derms, the madreporic body is internal, and there is a ca- 

 pacious cloaca or rectum, and a large vent. 



On the inside of the body-walls are numerous small cir- 

 cular (transverse) muscles forming slight ridges, which serve 

 to contract the body, and live double large longitudinal 

 muscles (Fig. 89, I) lying in the ambulacral zones. The 

 mouth is surrounded by a muscular ring, from which arise 

 ten large, much-branched tentacles. The pharynx, or the 

 portion corresponding to " Aristotle's lantern," of the sea- 

 urchin is broad and short, with five large retractor muscles 

 (r) originating from the ambulacral or longitudinal muscles 

 on the anterior third of the body. The stomach is short, 

 not much wider than the intestines, with well-marked trans- 

 verse folds within. The intestine (i) is several times longer 

 than the body, with longitudinal small folds, and held in 

 place by a large, broad mesentery which accompanies the in- 

 testine through the greater part of its length. The intes- 

 tine terminates suddenly, in a large cloaca (f), from which 



* In Mipyrgus and Echinocucumis it is difficult to perceivo any radia- 

 tion in the body except in tlie unbrolcen circle of tentacles, while in 

 Sipunculus and allied worms (Oephyrea) the tentacles fonn a complete 

 circle, and these worms have a ring-canal and an imperfect or rudi- 

 mentary system of vessels thought by some authors to correspond to 

 the water- vascular system of Echinoderms. 



