206 



THE EDRIOASTEROIDEA 



one may suppose it to have been derived. But the structure and 

 relations of the ambulacra, even in their least specialised form, at 

 once remove the type from primitive simplicity, and place it on a 

 road different from that traversed by other Pelmatozoa. The evi- 

 dence suggests the existence of a circumoesophageal water-ring, with 

 five perradial canals, and their associated nerves and blood-vessels, 

 passing between or below the thecal plates, and underlying a 

 ciliated food-groove, which was covered by an alternating series 

 of movable plates (covering-plates = ambulacrals of Crinoidea, but 

 probably not those of Echinoidea and Asteroidea). Pores between 

 the plates lining or flooring the groove (adambulacrals of Pelma- 

 tozoa, but perhaps = superambulacrals of Asteroidea) permitted 

 the passage either of podia or ampullae. In Cystidea, Blastoidea, 

 and primitive Crinoidea, on the other hand, there was a free exit 

 for the ambulacral organs only through the peristome ; in fact, 

 Blastoidea and Cystidea present no evidence that they possessed 

 perradial water-vessels and podia at all. 



FIG. I. 



Stromatocystis pentangular is, oral surface. 

 As, anus ; 0, peristomial plates ; c.p, covering- 

 plates ; s.p, side-plates ; ia, interambulacrals. 

 (Reconstructed from Pompeckj'sf figures.) 

 Slightly enlarged. 



FIG. II. 



Cystaster granulatiis, from pos- 

 terior, showing oral surface in per- 

 spective. The two left-hand rays 

 retain the covering-plates, which are 

 lost from the others. Lettering as 

 in Fig. I. (Reconstructed from Hall's 

 figures.) x 3 fliam. 



The primitive sack form did not long persist, but the follow- 

 ing characters were, as a rule, impressed upon it : a sessile habit, 

 the consequent assumption of a circular, flattened form, the differ- 

 entiation of the upper and under surfaces, the development of 

 marginals or concentric frame-plates, and the tendency to increase 

 the food-gathering surface by spiral coiling of the ambulacra in 

 either sinistral or dextral direction. According to the varied extent 

 of these several modifications, the Edrioasteroidea are divisible into 

 3 families Agelacrinidae, Cyathocystidae, Edrioasteridae. But 

 to these must be added a fourth, Steganoblastidae, in which the 

 development of a short stem was correlated with greater concentra- 

 tion and regularity of the thecal elements. 



