300 CYSTOIDEA, BLASTOIDEA, AND HOLOTHUROIDEA. 



other two larger and pentagonal. According to Lyon and 

 Billings, each of these three basals is really composed of an 

 upper and lower piece closely anchylosed together. Above 

 the basals (6) come five large plates (figs. 177, 17.8, d\ 

 which are usually called "radials," though it is not certain 

 that they correspond with the plates so called in the true 

 Crinoids. Each of these " radial " plates is rendered forked 

 or deeply cleft by the presence of a deep sinus or excavation 

 in it's upper margin. This sinus encloses the lower portion 



B 



Fig. 177. Structure of Blastoids. A, Calyx of Pentremites, dissected, showing the basals 

 (&), the forked "radials" (d), and the " deltoid " plates (i). B, Side view of the calyx of 

 Pentremites cervinusihe letters as before, c, Section across the calyx of Pentremites 

 ellipticus, showing the respiratory tubes cut transversely in their course below the 

 pseud -ambulacra. D, Section across one of the pseud-ambulacral areas of Pentremites 

 florealis, showing the compound nature of the respiratory tubes, enlarged. (After Hall 

 and Kofe.) 



of a pseud-ambulacrum, and these plates, from their shape, 

 are often spoken of simply as the " forked plates." Towards 

 the summit of the calyx the adjacent arms of the fork of 

 each contiguous pair of " radial " plates are separated to 

 receive one of a series of much smaller rhomboidal pieces, 

 which are thus five in number, and are known as the 

 "deltoid" plates (figs. 177, 178, i). Between the "del- 

 toid " plates and the forked ends of the " radials " are sit- 

 uated the "pseud-ambulacra." These (figs. 177, 178, p) 

 are broad petaloid areas which pass in a radiating manner 

 from the centre of the summit of the calyx to its margins. 

 It is these which give to the summit of the body its resem- 

 blance to a flower-bud, upon which the name of the order is 

 founded (Gr. llastos, a bud ; and eidos, form). The petaloid 

 pseud-ambulacra are not particularly well named, as it is 



