458 PELMATOZOA. 



incised distally, in such a way that the two arms of the plate 

 enclose the lower end of one of the ambulacral fields. The radials 

 differ from the corresponding plates of the Crinoids in the import- 

 ant fact that they do not form the starting point for a crown of 

 "arms." Placed towards the summit of the calyx, at the angles of 

 junction of the radials, are five triangular, rhomboidal, or pen- 

 tagonal interradial plates, which have commonly been spoken of as 

 the " deltoids " (fig. 327, a and b, i). The cycle of the deltoids is 

 interrupted by the ambulacra, and a considerable portion of their 

 entire area may be hidden from view beneath the adjoining plates 





A 



Fig. 327.— Structure of Blastoids. a, Calyx of Pentremites, dissected, showing the basals (£), 

 the forked "radials" (d), and the interradial or "deltoid" plates (z). b, Side-view of the calyx 

 of Pentremites cervinus — the letters as before, c, Section across the calyx of Granatocrinus 

 ellipticus, showing the hydrospires cut transversely in their course below the ambulacral areas. 

 d, Section across one of the ambulacral areas of Pentremites Godoni, showing the compound 

 nature of the hydrospires, enlarged. (After Hall and Rofe.) 



of the ambulacral fields. The " deltoids " of the Blastoids cor- 

 respond very closely with the large interradials of Cyathocrinus, as 

 shown in fig. 297, and, through these, with the interradial systems 

 of the other Crinoids. 



The most complicated structures in the test of the Blastoids are 

 those connected with the ambulacral areas. The ambulacral fields 

 (formerly spoken of as the " pseudambulacra ") are always five in 

 number, and 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 resemblance to a flower-bud (figs. 327, b, and 

 328, a) upon which the name of the class is founded (Gr. blastos, 

 a bud ; and eidos 1 form). Though usually broad and petaloid in 

 shape, the ambulacra are sometimes narrow, and their length varies 

 in different types of the class. In a typical Blastoid, such as a 

 species of Pentremites, the ambulacra have the following structure. 

 Running down the centre of each of the petaloid ambulacral fields 

 is a well - marked median groove, often with crenulated edges 

 (fig. 329, b, ag), which in perfect specimens is roofed over by a 

 double row of small calcareous ossicles or "covering-plates (fig. 329, 



