lOG WYVILLE THOMSON, ON ASTERACANTHION VIOLACEUS. 



of one brood, reared in a jar in a warm room, the temporary 

 appendage entirely disappeared, the rudiments of the princi- 

 pal plates on both surfaces of the embryo assumed a definite 

 arrangement, and the permanent mouth of the star-fish was 

 formed between a fortnight and three weeks after the seg- 

 mentation of the yelk. In other instances, however, develop- 

 ment took place more rapidly, the greater rapidity depending, 

 I believe, upon a more plentiful supply of organic matter in 

 the water. All the broods reared in the house left the mar- 

 supium early, while the peduncle was still attached ; but in 

 other specimens which I have procured from time to time 

 from pools at low- water mark, the pouch has been filled with 

 fully formed embryos with the peduncle entirely gone, six 

 sucking feet on each ray, and the permanent digestive system 

 of its normal character. 



It would appear from the foregoing observations, that the 

 first step in the development of this form of Echinoderm 

 embryo is the differentiation of a portion of the yelk into an 

 investing layer of structureless '^sarcode;^^ that the layer 

 gradually increases in thickness ; and that, finally, from one 

 part of its surface a branched peduncular process is produced 

 as an extension of the same transparent structureless material. 

 The branches of this organ are terminated by suckers, and 

 serve, among other functions, as organs of locomotion. When 

 fully formed they are undistinguishable in structure and 

 function from the ambulacral feet of the star-fish; a fluid un- 

 distinguishable from the chylaqueous fluid of the ambulacral 

 system moves in them with the same characteristic motion. 

 The peduncle is closed externally, no communication except 

 by transudation existing between its cavity and the sur- 

 rounding medium. At first it communicates with the general 

 cavity of the embryo, but afterwards it becomes connected 

 with, and part of, the ambulacral circulating system. When 

 the ambulacral vessels and suckers of the young star-fish 

 become fully developed, this provisional vascular tuft withers 

 and disappears, leaving no apparent scar. 



In the species described, the peduncle is not connected in 

 any way with the madreporic tubercle, which is not developed 

 till long after its disappearance, and then on the opposite sur- 

 face of the body. 



I believe this peduncular appendage to be essentially a 

 provisional development of the ambulacral vascular system, 

 and to be functionally analogous, not to the vitelline sac, 

 but to the omphalo-mesenteric and umbilical vessels of the 

 higher groups. 



I believe, hoAvever, that it is endowed with a greater 



