108 WrVILLE THOMSON; ON ASTERACANTHION VI0LACEU8. 



sarcode, and indicating a surface especially dedicated to 

 assimilation. 



By a third modification, of which Aster acanthion viola- 

 ceus is an example, a portion of the sarcode layer is deve- 

 loped into a group of transparent tubes, which act as a tem- 

 porary assimilative and respiratory apparatus; and in a 

 fourth series, e. g. in the species producing the " Bipinnaria^' 

 and " Brachiolaria^^ larvae, the whole of the segmented yelk, 

 or the whole with the exception of a small granular nucleus, 

 is shaped into a mass of sarcode, which forms a complicated 

 organ provided with locomotive and respiratory appendages, 

 a mouth, and short intestine; eventually, however, this or- 

 ganism declares precisely the same relations to the embryo 

 as in the former case, withering finally, as a cast-off pro- 

 visional appendage of its ambulacral system. 



Regarding the embryonic appendages in the Echinoderms 

 as a provisional development of the ambulacral vascular 

 system, the analogy between them and the embryonic vas- 

 cular appendages of the vertebrata becomes extremely simple. 

 In the higher group, simultaneously with the first appear- 

 ance of the embryo, a temporary system of vessels is pro- 

 duced for its nourishment. These vessels originate round 

 the outer edge of the area vasculosa, at some distance from 

 the embryo and quite distinct from it, though in a continua- 

 tion of the same layer of segmented yelk (the germinal 

 membrane). They then approach the embryo, uniting and 

 forming two or more symmetrical vascular trunks, which at 

 first seem to open into the general cavity of the embryo, but 

 afterwards coalesce with its special vascular system. 



The minute linear embryo presents at this time a form 

 quite as anomalous as that of the most aberrant Bipinnaria 

 or Pluteus ; flanked on either side by a large, crescentic, 

 vascular lobe. As development advances, the vessels are 

 carried backwards from the embryo with the nascent germinal 

 membrane till the whole yelk is inclosed is a delicate, anas- 

 tomosing, absorbent network ; other vessels and tissues are 

 subsequently formed, but it is unnecessary here to trace 

 their complicated morphology. 



With reference to the earliest stages it has been already 

 shown that almost the same language would apply to Echino- 

 derm development. 



