THE BLASTOIDEA 



that the object was an increase of surface for respiratory purposes, the 

 outer oxygenated sea-water passing down into the folds and the inner 

 coelomic fluid passing up into the alternate folds. Hence to such struc- 

 tures E. Billings gave the name "hydrospires." They are similar, in 

 essential structure and in position across the suture lines, to the pectini- 

 rhombs of Callocystinae ; and similarly are a development of the normal 



FIG. V. 



Cadaster trilobatus. 1, oral surface of young individual of var. acutus, with 3 hydrospires 

 each side of a food-groove and traces of them in anal interradius (Brit. Mus. E8020). 2, a theca 

 ground down from the oral surface, thus bringing out the sutures (Brit. Mus. E8023). 3, 

 analysis of the main thecal elements. 4, theca seen from the left posterior radius. 5, the 

 same theca from below, with posterior interradius uppermost. (Both from Brit. Mus. E8013). 

 x$. 6, slightly restored section across part of a radius, amb, so-called ambulacrum or 

 pseudambulacrum, with food-groove ; As, aperture for anus ; B, basal ; br, brachiole ; c.p, 

 covering-plates ; h, hydrospire-folds ; L, lancet-plate ; 0, mouth-aperture ; R, radial ; s.p, side- 

 plate ; A, deltoid. 



structure of the test. The marked pentamerous symmetry of the thecal 

 plates (with the apparent exception of BB) and of the hydrospires is 

 disturbed only by the anus, which makes an opening between the pos- 

 terior A and the adjacent radial processes. From this IE. hydrospire-folds 

 are said to be absent (but one or two may be seen in some specimens, 

 Fig. V. 2). The anal opening was closed by small plates. The essential 

 structures of Cadaster are all to be found in Protoblastoidea ; the absence 

 of interambulacrals, in Blastoidocrinus ; the ambulacral structures, in 



