ZOOLOGICAL Cfl LttAOTEBS, 117 



another and to the Urchins or Starfishes. This is also the classification which was 

 adopted in the 'Challenger' Report, and will be employed in this Catalogue. 

 During the earlier portion of our joint work on the Blastoidea it appeared to us that 

 the group had several affinities with the Cystids, and it wasstated by one of us ■ that 

 they are "so closely linked together that it is extremely difficult to refer forms like 

 HybocyaMtes and Oystoblastus to one group rather than to the other." 



Of late, however, we have been led to the conclusion that the group Blastoidea is 

 in reality an extremely well defined one, and that there is perhaps a closer affinity 

 between Cystids and Crinoids through Porocrinua than between the former group 

 and the Blastoids, no really intermediate type being known to us. 



The following definition embraces the result of our seven years' study of the group. 

 It is preceded by a definition of the Pelmatozoa which is based on that published in 

 the ' Challenger ' Report 2 , and subsequently modified by Wachsmuth and Springer ; . 



Phylum ECHINODERMATA. 



Branch Pelmatozoa. 



Echinoderms which are fixed either permanently or temporarily by the middle of 

 the aboral surface. A jointed stem containing a neuro-vascular axis is usually 

 present, but may be lost wdien maturity is reached ; or, in the case of a few sessile 

 forms, remain altogether undeveloped. The apical system consists of a dorsoccntral 

 plate, basals, and radials, with the frequent addition of under-basals and interradials. 

 These plates form a cup, which either simply supports, or more or less completely 

 encloses, the visceral mass, and often bears jointed appendages — the arms and 

 pinnules. 



An oral system, which is to some extent a repetition of the apical system, and 

 consists of a central plate, basals, radials, and interradials, is developed to a very variable 

 extent above the actinal surface of the larva. It may be (1) altogether resorbed ; 

 (2) persist as basals only which cover the peristome ; or (3) reach a high state of 

 development so as to form a complex vault or tegmen cah/cis covering in the whole 

 visceral mass. The anal tube opens on the oral surface, but is sometimes in close 

 relation with the calyx-plates. 



The water-vascular ring does not communicate directly with the exterior, and the 

 lateral branches of the radial vessels (when present) are respiratory, but not loco- 

 motor in function. 



Class Blastoidea. 

 Armless Pelmatozoa of a pyriform, clavate, ovate, or globose shape, which usually 

 exhibits a very perfect radial symmetry. Base monocyclic, of two large plates and 

 1 Zool. Chall. Exp. Part xxxii. 1884. p. 101. 2 Ibid. p. 186. 



3 ' Revision of the Palteocrinoidea,' Part III. 1885, p. 79. 



