176 LOWER INVERTEBRATES. 



shaped suckers. The excretory opening is at the posterior extremity. Only two 

 ovaries are developed, and the eggs appear to be very large at the time of expulsion, 

 corresponding to the large size of the ovarial openings. 



The genus Schizaster includes several species of almost circular outline when 

 viewed from above, but with a deeply sunk anterior ambulacrum, and the other ambu- 

 lacra depressed. In the young the odd anterior ambulacrum, as in Aceste, occupies 

 the greater part of the upper surface, and the suckers of this ambulacrum are very 

 large. Thus, Aceste may be regarded as a permanent form of the young of Schizaster. 

 Other Brissina are the well-known large species Jirissus carinatus, which is widely 

 spread in the Pacific, and reaches a length of seven inches, and a width of nearly six ; 

 the pretty little egg-like and delicate Agassizia scrobiculata of the west coast of 

 Mexico, and T'erinopsis lyrifera of European seas. 



Class IV. — HOLOTHUEOIDEA. 



The Holothurians or Sea Cucumbers are the least radiate and least typical of 

 echinoderms, approaching the worms in the length and usually cylindrical form of the 

 body (which is elongated in the direction of the axis), of the oral and aboral systems, 

 and is without arms. The mouth is surrounded by a circle of branched tentacles, and 

 the body-wall is muscular and leathery, instead of presenting a calcareous test or 

 system of calcareous plates or ossicles, as is the case with other echinoderms. But 

 though there is usually no continuous calcareous armature, the integument contains 

 numerous calcareous bodies of varying form. The body-wall consists of an external 

 skin, within which is a layer of connective tissue, and inside this a layer of muscular 

 fibres, some of which are disposed in circles around the body, while others form five 

 longitudinal bands. These bands are attached to a calcareous ring surrounding the 

 mouth. The calcareous plates forming this ring are ten or twelve in number, and the 

 longitudinal muscles are attached to five of these. These five plates are also notched 

 for the passage of the nerves and ambulacral canals. 



The water-system varies greatly in degree of development. In one section of the 

 class (Apoda) there are no ambulacral feet or ambulacral canals, and the only trace 

 of the water-system is to be found in the ring-canal round the gullet, with its gi-eater 

 or less number of Polian vesicles internally, and the circlet of comparatively simple 

 tentacles externally, and in the madre;poric or stone canal or canals, which run down- 

 wards into the body and terminate in a calcareous network, the homologue of the 

 madreporic bo(}y of other echinoderms. In all the class, with two exceptions, the 

 madreporic body is internal, and by its openings the water-system communicates, not 

 with the external water, but with the large bod3'-cavity that intervenes between 

 the intestinal canal and the body-wall. 



In the higher Holothuria or Pedata the circular vessel of the ambulacral system 

 not only gives origin to the Polian vesicles, stone-canals, and tentacles, but to five 

 ambulacral canals, which pass through holes or notches in the plates to which the 

 longitudinal muscles are attached, and run backward along the median line of each of 

 those muscles, immediately interior to the longitudinal nerve. In most cases each of 

 these ambulacral vessels is furnished with ampullae, connected with processes of the 

 body-wall which form suckers or ambulacral feet, much as in a sea-urchin. In other 

 genera some of the rows of suckers are suppressed, the other rows forming a surface 

 upon which the animal creeps. 



