CHAPTER X. 



THE BLASTOIDEA. 1 



CLASS II. BLASTOIDEA, SAY (1825, sensu extenso). 



GRADE A. Protoblastoidea. 

 B. Eublastoidea. 



PELMATOZOA in which five (by atrophy four) epithecal ciliated 

 grooves, lying on a lancet-shaped plate (? always), radiate from a 

 central peristome between five interradial deltoid plates (A), and 

 are edged by alternating side-plates bearing brachioles, to which side- 

 branches pass from the grooves. Grooves and peristome protected 

 by small plates, which can open over the grooves. The generative 

 organs and coelom probably did not send extensions along the rays 

 into the brachioles ; but apparently nerves from the aboral centre, 

 after passing through the thecal plates, met in a circumoral ring, 

 from which branches passed into the plate under the main food- 

 grooves, and thence supplied the brachioles. The thecal plates, 

 however irregular in some species, always show defined basals (B) 

 and a distinct plate (" radial," R) at the end of each ambulacrum ; 

 they are in all cases so far affected by pentamerous symmetry 

 that their sutures never cross the ambulacra. 2 



The more primitive of these forms can hardly be distinguished 

 from their immediate ancestors among the Cystids, such as Pro- 

 teroblastus and Mesocystis, except by the more developed basals and 

 radials ; and it is this greater intimacy of correlation between 

 ambulacral and thecal structures that necessitates their removal 

 from the class Cystidea as here defined. Those general relations 

 of the ambulacra to the theca, shared by Blastoidea with Diplo- 

 porita, serve to distinguish them from the Callocystinae, with 

 which some of the genera have been allied by naturalists. To 

 these characters may be added the presence of diplopores, which 

 are still to be found in the most primitive genus. From the 



1 By F. A. Bather, M.A. 



2 The term "ambulacrum" has been loosely used in the Blastoidea for the 

 thecal elements connected with the food-groove. " Pseudambulacrum " is more 

 correct and more cumbrous. The relations of the true ambulacral system are doubtful. 



