330 COMPARATIVE ANATOMY chap. 



always of unequal size, and it appears not unlikely that the reduction 

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

 These characteristics necessarily destroy the strictly radial symmetry 

 of the dorsal cup. 



Still further fusions may occur (among the Canaliculata). 



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 



Jfy irregular Blastoids, on the 



pi/ contrary (Elentherocrinus, As- 



^ ^ trocrinus, Fig. 267, p. 315), 



_ " the radiate structure is dis- 



_^^^ ^^^f turbed by the modified form 



^^^^^H Ri' \~-> ^^^^^^ °^ "-"^^ °^ ^^® ambulacra. 



^^^^^»^^^^ The outline of the ovoid 



^^.^^ .A^^.^^. ^<x body 01 Lleutlierocnnus, seen 



/"^ 7 ^KKI '•■ ^1^^ ^^^"~^\ ^i"o™ ths apical or oral pole, 



U^ Wf \ ^K^^ '® irregularly pentagonal, 



U ^^ with three shorter and two 



i longer sides, the latter be- 



Cjy longing to the left posterior 



) i ( and the unpaired posterior 



V interradii. In Aitrocmiu% 



! the body is flattened in the 



OB ' ^ 



^^ direction of its jjrincipal axis, 



Fic. 292. -Apical system otPentremites. ,mM>, Axis and, when seen from the oral 



passiit'; thi'outrli jnouth and aims ; .'■, the smaller : w, and ^ ^ ^ ^ l 



z, the two larger basals ; ir, interradials ; r, radials. Or ^boral 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 plates (Fig. 292), two of 

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



