200 ROBERT TRACY JACKSON ON ECHINI. 



approach to other classes of the Echinodermata. Attempts have been made to connect the 

 Echini with the Asteroidea, especially through the genus Palaeodiscus, as later discussed, but 

 the present evidence is opposed to this view. Mr. A. H. Clark (1909) has recently attempted 

 to connect the Echini with the Crinoidea. In Echini all plates of the corona originate beneath 

 a permanent plate, the ocular, as they do not in crinoids. The reproductive bodies of Echini 

 are internal and interradial in position, whereas in crinoids they are situated on the arms. 

 The peristome of Echini (excepting the Exocycloida) bears ten or more ambulacral plates, as 

 the oral region of crinoids does not. Clark has attempted to correlate the auricles of Echini 

 with ambulacral plates of crinoids. Auricles are unknown in Palaeozoic Echini and the Cida- 

 roida; they exist only in the Centrechinoida and some Exocycloida, both specialized types. 

 Moreover, they do not occur (Centrechinoida) at the ventral end of the ambulacra as Clark's 

 theory calls for, but very far from the ventral termination of the ambulacra, which is the row 

 of primordial ambulacral plates around the mouth. This is best seen in Phormosoma (text- 

 fig. 226, p. 193), or other echinothuriids, where many rows of ambulacral plates exist between 

 the auricles and the mouth. What the ancestor of the Echini as a class was is unknown, but 

 it might fairly be sought amongst the Cystoidea. 



The Echini, though possessing a wide range of structure, may be described as animals 

 possessing alimentary, reproductive, nerve, and water vascular systems within an enclosing 

 superficial pentamerous skeleton which bears movable spines. There are from two to twenty 

 columns of plates in each of the five ambulacral areas and from one to fourteen columns of 

 plates in each interambulacral area. New coronal plates are formed at the ventral border 

 of the five ocular plates, ambulacral pores pass through ambulacral plates, rarely (clypeastroids) 

 in part between plates. The peristome in all but the Exocycloida bears from one to many 

 rows of ambulacral plates, with or without non-ambulacral plates. There are five oculars 

 (apparently in part or wholly wanting in some of the Pourtalesiidae), and five genitals or fewer, 

 the whole being fused into a mass in certain types of Exocycloida. The genitals typically 

 have each one or more pores as exits of the five interradially situated reproductive glands. 

 In addition, typically, madreporic pores exist in genital 2, but are not recognizable in most 

 Palaeozoic forms. The periproct is more or less plated, situated within the oculo-genital ring, 

 or in irregular types outside of that area; the anus is in the periproct. The masticatory lantern 

 is composed of forty pieces (or clypeastroids thirty pieces) ; it is wanting in adult spatangoids. 

 Respiratory organs consist of Stewart's organs, peristomal or ambulacral gills. Locomotion 

 is effected by ambulacral feet or by spines, or both. 



Such being the character of the class as a whole, the following key presents the characters 

 by which the several groups may be divided. It is too much to hope that errors of diagnosis 

 will have been entirely overcome, but it is felt that it is an approximation to the real affinities 

 as gathered from a comparative study of the characters of young, adult, fossil, and living types. 



