378 



p. HERBERT CARPENTER. 



by the primitive digestive sac. This is also the condition 

 of the Holothurian Gastrula, as figured by Krohn and 

 Selenka. But the Gastrula of a Crinoid differs extremely 



I'iG. X. — Echinoderm Gastrulae. 

 B. Comatula. (After Gotte.) 



A. Aster acanthion. (After Agassiz.) 

 hi. Blastopore. 



in its shape from that of any other Echinoderm, for it is 

 very much elongated transversely to the axis of the diges- 

 tive sac, which is comparatively short (fig. x, b). Conse- 

 quently, in the Crinoid Gastrula the principal mass of the 

 body lies at the sides of the archenteron, and not in a line 

 with it as in other Echinoderm Gastrulae. This extreme 

 elon^^ation of the transverse axis provides the space for the 

 development of the future stem, which is so characteristic 

 of the Crinoids. It first appears (fig. viii) as a series of 

 delicate calcareous rings, that form a curved line passing 

 backwards from beneath the centre of the ring of basals, 

 behind and slightly to the left of the large keyhole-shaped 

 depression of the outer surface of the body, at the narrow 

 anterior end of which is the Gastrula mouth or blastopore (5/). 

 As development proceeds this depression becomes obliterated, 

 and the stem lengthens very considerably, so that the part of 

 the embryo body behind^ the blastopore (which indicates the 

 position of the future anus), becomes very much longer than 

 the corresponding part in front of the blastopore with which, 

 in the Gastrula stage, it was symmetrical. In correspondence 

 with the extreme elongation of that part of the body of 

 the larval Comatula which lies behind the blastopore 

 (fig. XI, st), the right peritoneal tube (fig. xi, rp) sends a 

 backward process into the mass of mesoblast of which it is 

 mainly composed {rp'), and it is around this tubular process 

 that the primitive rings representing the future stem seg- 

 ments are developed (fig. xi, st). 



' The term behind is here used to indicate the opposite end of the trans- 

 verse axis of the Gastrula to that at which the future mouth appears, 



