408 



CHAPTER XXV. 



DIVISION B. PELMATOZOA. 



The three groups of Echinoderms known as the Crinoids, Cystoids, 

 and Blastoids agree with one another in certain common characters, 

 and may be included in a single primary section termed Pelmatozoa. 

 In all these forms the body is fixed, either temporarily or perman- 

 ently, by the dorsal surface, often having a jointed stem or peduncle ; 

 and the mouth is placed on the opposite side of the body. In its 

 fully developed condition, the peduncle has the form of a jointed 

 stem, containing a neuro-vascular axis in its interior. The body 

 itself is enclosed in a variously modified series of calcareous plates, 

 which represent the apical disc of the Echinoids, and the upper 

 surface may be provided with jointed appendages (the " arms "). 

 The circular ambulacral vessel has no direct communication with the 

 exterior, or only a limited one, and the radiating ambulacral vessels 

 (when present) are respiratory in function, and are not subservient to 

 locomotion. 



Class I. Crinoidea. 



The Crinoids or " Sea-lilies " may be defined as Echinoderms in 

 which the body is fixed, during the zvhole or a portion of the existence 

 of the animal, to the sea-bottom by ??ieans of a jointed, flexible stalk or 

 peduncle, which springs from the centre of the dorsal or aboral surface. 

 The body is cup-shaped or discoidal, and its dorsal surface is protected 

 by a system of calcareous plates. The mouth is situated on the upper 

 surface, generally in the centre. From the margin of the cup-shaped 

 body spring jointed flexible appendages or "arms," which are primi- 

 tively five in number, and which carry lateral jointed processes or 

 "pinnules," The upper or ve?itral surfaces of the arms are furnished 

 with grooves corresponding with the " ambulacral grooves " of the 

 Asteroids. The ivater-vascular system has only a limited communica- 

 tion with the exterior^ aitd is not connected with locomotion. The 



