TYPE ECHINODEEMA. 585 



but feebly developed in the Holotburians, the body-walls 

 having as a rule a somewhat leathery consistency, though in 

 Elpidia interlocking spicules, and in Deima and Psoitis closely- 

 approximated or overlapping plates, give the skin considerable 

 rigidity. In the majority of forms, however, the calcareous 

 skeleton is represented by scattered plates of various shapes, 

 perforated, knobbed, sometimes wheel-shaped as in Ohirodota, 

 or associated with an anchorlike spicules in Synapta, and 

 are not sufficiently numerous to give a rigidity to the integu- 

 ment, which in Synapta may even be thin and translucent. 

 There is no indication of an apical system of plates, though in 

 Mulleria a circle of five plates surrounds the anus, but the 

 oral system is represented in a species of Psolus by five 

 plates which may be closed over the mouth and tentacles. 

 In other parts of the body than the integument calcareous 

 matter is also frequently deposited, especially in the connec- 

 tive tissue of the wall of the peripharyngeal cavity, the pharyn- 

 geal ring (Fig. 269, h) so formed consisting typically of five 

 radial ossicles, grooved or perforated by the radial nerves 

 and hydrocoel-canals, and of five interradial ossicles alternat- 

 ing with them, though in those forms in which the number of 

 tentacles is greater than ten the number of the interradial 

 ossicles may be increased. Spicules are also found in the 

 mesentery, and in some forms plates or a calcareous network 

 develops in the wall of the pharynx. Spines are rarely 

 present, though the plates of EcMnocucumis, Elpidia, and a few 

 other forms bear them, and pedicellarise are entirely wanting. 

 The ccelom is traversed by several mesenteries uniting the 

 digestive tract to the body-wall, the most constant being the 

 so-called dorsal mesentery (Fig. 269, m), which lies in the 

 anterior portion of the interradius CD. The portion of the 

 coelom which surrounds the oesophagus is separated from the 

 rest, as in the Echinoidea, and forms the peripharyngeal space, 

 and similarly in some forms {Cucumaria, Holothuria) a perianal 

 space surrounds the terminal portion of the digestive tract. 

 In Synapta and its allies there are attached by slender pedun- 

 cles to the body-wall along the line of the mesenteries, and 

 hanging freely in the body-cavity, numerous ciliated urnlike 

 bodies, which probably function similarly to the ciliated cups 



