PROFESSOR WYYILLE THOMSON OH THE ECIIIHODERMATA. 401 
Holothuridea, seem conclusive that this pore and canal are the first 
indications of the madreporic tubercle and sand-canal of the Echino- 
derm. 
14. Soon after these parts have been formed, a definite hyaline 
mass, containing multitudes of granules and oil-cells, stretches over 
the posterior surface of the stomach. It gradually forms a rounded 
cake, falling over the sides of the stomach, and at length partially in¬ 
volving it like a cowl. This is the nascent perisom of the starfish. 
At the proximal end of the dorsal canal, at the top of the flask¬ 
shaped sac, and at the right side of the stomach of the pseudembryo, 
a rosette of five minute vascular loops, or coeca, may now be traced. 
These are the rudiments of the central ring of the ambulacral vascu¬ 
lar system. The granular hood passes downwards beneath the integu¬ 
ment of the pseudembryo, and covers the back of the stomach as 
far as the dorsal pore, it involves the pore and canal, and passes 
round the oesophagus, so that the stomach of the pseudembryo and 
the vascular rosette are at length included within it, and a raised 
zone or border passes round the now nearly spherical granular hood. 
This border is the first indication of the margin of the disk of the 
starfish, and it defines the dorsal and ventral surfaces of the disk. 
It passes round the central granular mass, not symmetrically but 
obliquely, so that, at length, when the rim is fully formed, the star¬ 
fish looks like a little cap set obliquely on the end of the pseudem¬ 
bryo. Shortly a series of elegantly formed calcareous spiculse appear 
round the lobed border, and nascent spines and stars of calcified 
areolar tissue define more fully the relations of the various parts. 
As in JBipinnaria asterigera , the dorsal surface of the disk of the 
starfish is turned obliquely towards the centre of the posterior ex¬ 
tremity of the pseudembryo, and the relations of the oesophagus, 
stomach, intestine and anus, and of the madreporic tubercle and sand- 
canal, are the same as in the previously described form. 
15. Itesuming these various observations on pseudembryos of 
the “ Bipinnaria ” type, we find that the general structure is essen¬ 
tially the same in all. The earliest stages in the development of these 
forms have not as yet been observed, but judging from the analogy 
of the development of the closely allied “ Pluteus ” pseudembryos of 
the sea-urchins, # there can be no doubt that, after impregnation of 
the egg and complete segmentation of the yelk, the whole germ-mass 
d. pseudoproct ; e. ciliated sac, with corpusculated fluid contents ; f. rosette of vas¬ 
cular loops, the rudiment of the ambulacral vascular ring of‘ the Echinoderm. 
g. Ventral fold of the dorsal integument of the pseudembryo. 
h. Border with calcareous spicuke, indicating the margin of the disk of the star¬ 
fish. (After Muller.) 
* Derbes, “ Observations sur le mecanisme et les phenomenes qui aceompag- 
nent la formation de Pembryon chez foursin comestible .”—Ann ales des Sciences 
Naturelles, 1847. 
Krohn, “ Beitrag, zur Entwickelungsgeschichte der Seegellarven.”—Heidel- 
