144 



ECHINODEEMATA 



PHYLUM IV 



behaves miiieralogically and optically like a single calcite crystal. The plates 

 forming the main skeleton of an Echinoderm may be few or numerous, and 

 may be polygonal with vertical sides forming a solid skeleton, or they may 

 be rounded, scale-like or imbricating, forming a more or less flexible test ; or 

 again they may be reduced to minute, dissociated bodies embedded in the 

 integument and forming a partial dermal skeleton, as in certain Holothurians. 

 All the Echinoderms are marine, and only a very few of them occur in 

 even very slightly freshened water. In the system proposed by Haeckel 



Fig. 226. 



(i, Magnified cross-section of an 

 Echinoid spine ; Fiji Islands. /), Mag- 

 nified section of coronal plate of a 

 Recent Sea - urchin {Sphaercchinus) ; 

 plane of section jjarallel to surface. 



Fif!. 227. 



Pentitcrinus suhtcres Goldf. Upper Jura ; Keichen- 

 bach, Wurtemberg. a, Vertical section of stem-joint in 

 plane indicated in c, IS/j. b, Transverse section of 

 same, IS/j. c, Joint-face, d, Series of columnals 

 (natural size). 



seven classes are recognised, of which the first three, namely, Cystidians, 

 Blastoids and Crinoids, are grouped together as a distinct subphylum called 

 Felmatozoa. Corresponding to this are two other subphyla, Astero::oa and 

 Echinozoa, the former including the classes of Asteroids and Ophiuroids, 

 and the latter comprising the classes of Echinoids and Holothurians, 



Subphylum A. Pelmatozoa Leuckart. 



The Pelmatozoa are Echinoderms, nearly all of which, during the whole or 

 at least the early portion of their existence, are fixed by a jointed, flexible 

 stalk, or are attached by the dorsal or aboral surface of the body. The prin- 

 cipal viscera are enclosed in a bursiform, cup-shaped or spherical test (calyx), 

 which is composed of a system of calcareous plates. On the upper surface of 

 the test are placed both the mouth and anus, as well as the ambulacral or 

 food grooves conducting to the mouth. In some forms, however, the calyx is 

 so reduced as to form merely a small horizontal platform upon which rest the 

 viscera, usually protected by a covering of secondary dermal ^^lates. As a 



