THE ANNALS 



▲ NO 



MAGAZINE OF NATURAL HISTORY. 



[SECOND SERIES.] 

 No. 76. APRIL 1854. 



XXII. — On the Structure of the Echinodenns. 

 By Johannes Muller. 



[Concluded from p. 123.] 



Crinoidea. 



Nature has produced no transitional form between the Sea- 

 urchins and the Starfish, which would be a flattened Sea-urchin 

 with an ambulacral abdominal surface and an entirely antambu- 

 lacral dorsal surface, but ^"ith only the double series of interara- 

 bulacral plates of the Sea-urchins. The sole approximation to 

 this form is the pentagon of the pentagonal kinds of Starfishes, 

 whose interambulacral plates always form a triangular accu- 

 mulation, of which only those plates which border upon the 

 ambulacrals are arranged in a similar order to these. Much 

 nearer the Sea-urchins in form, but not in composition, are, 

 among the Crinoids, the Blastoidea possessing a solid shell and 

 no free arms, especially those Pentremites* with rounded calyces, 

 and the genus Eleeocrinus, Roemer. The apex has enlarged into 

 the antambulacral area of the calyx. However, the composition 

 of the interambulacral arese of the calyx departs far more widely 

 from that in the Sea-urchins than these do from the Starfishes ; 

 in the Blastoidea these arese are formed partly by the five ra- 

 dialia which are disposed in the direction of the radii, partly by 

 the interambulacral azygos deltoid pieces, a conformation which 

 can be compared to nothing in the interambulacral areJB of the 

 Sea-urchins. The composition of the ambulacra is also aberrant in 

 the Pentremites, as well from those of the Asterida as from those 



* See, for an elaborate comparison of Pentremites with Asterida and 

 Ophiurida, Prof. E. Forbes's Memoir on the British Cystidea, Mem. Geol. 

 Survey. — Transl. 



Ann. &; Mag. X. Hist. Ser. 2. Vol. xiii. 16 



