330 



COMPARATIVE ANATOMY 



always of uneciiial size, and it appears not uulikelj' that the reduction 

 of their number was caused by the fusing of neighbouring plates. 

 These characteristics necessarily destroy the strictl}' radial symmetry 

 of the dorsal cup. 



Still furtlier fusions may occur (among the Canalicidata). 



The relative sizes of the plates of the infrabasal, basal and radial 

 circles vary greatly, but this is of no great interest to the comparative 

 anatomist. 



Sub-Class 2. Blastoidea. 



The Blastoidea are palaeozoic Pelmatozoa, whose stalked armless 

 body very often has the appearance of a bud (Fig. 263, p. 314). 

 Seen from the side, the body is an oval, truncated sometimes at 

 the apical, sometimes at the oral end. Seen from the oral or 

 aboral pole, its outline is in by far the greater number of (regular) 

 forms regularly pentagonal with rounded projecting angles, some- 

 times not unlike a short- 

 armed Star-fish (Figs. 265 

 and 266, p. 314). In the 

 irregular Blastoids, on the 

 contrary (Eleiiflterocrinus, As- 

 trocrinus, Fig. 267, p. 315), 

 the radiate structure is dis- 

 turbed by the modified form 

 (»f one of the ambulacra. 

 The outline of the ovoid 

 body of Eleuthcrocrimis, seen 

 from the apical or oral pole, 

 is irregularly' pentagonal, 

 with three shorter and two 

 longer sides, the latter V)e- 

 longing to the left jjosterior 

 and the unpaired posterior 

 interradii. In Asfrocrinu.'^, 

 the body is flattened in the 

 direction of its principal axis, 

 and, when seen from the oral 

 or aboral pole, almost sym- 

 metrically four-lobed, the 

 lobes being of unequal size. The largest of the lobes lies diametrically 

 opposite the abnormally shaped ambulacrum, which is on the smallest 

 truncated lobe. The two other middle-sized lobes are almost alike in 

 form (Fig. 267, p. 315.) 



The whole body of the Blastoids is plated. The test consists, 

 apart from the ambulacra, of three circles of })lates (Fig. 292), two of 

 Avhich belong to the typical apical system of the Echinodermata, while 



Fio. 292.— Apical system of Pentremites. im-lih, A\is 

 passing tliroiigli niimtli ami anus ; ./', tlie smaller ; //, ainl 

 J, the two larger basals ; ir, iiiterradials ; /■, radials. 



