STUDIES IN THE EMBEYOLOGY OE ECHINODEEMS. 413 
he describes the anterior enterocoels as “ Wassergefassan- 
lagen” (= hydrocoels), which I shall presently show to be 
incorrect. 
But whatever name we may give to these cavities I am 
entirely in agreement with Metschnikoff as to the reality of 
their existence, which has been frequently questioned, or even 
denied. Thus Gotte (11, p. 609) denies their presence in 
Spatangids, and asserts that the two primary enterocoel cavi- 
ties simply shift their position down to the stomach, while part 
of the left one at the same time forms the hydroccel. This 
view is also taken by Ludwig (17, p. 141), and apparently by 
Selenka (26), who does not otherwise account for the fact that 
the only two cavities noticed by him are at first situated beside 
the oesophagus, and afterwards beside the stomach. 
Though Metschnikoff’s account of this stage is therefore the 
most correct one yet given, yet in the next stage he has, as in 
his account of Ophiurids, fallen into error by confounding the 
hydrocoel with the anterior enterocoel ; it must, however, be 
admitted that the phenomena are here even more difficult to 
make out, and that a stage is soon reached in which, but for 
the evidence afforded by the Ophiurids, the fundamental dis- 
tinction between the anterior enterocoel and the hydrocoel could 
hardly have been recognised. 
In fig. 8 is represented the earliest stage in which I have been 
able to determine the relations of the hydrocoel. It consists, 
as in Ophiurids, of a vesicle with thick and well-defined walls, 
lying between the anterior and posterior enterocoels on the left 
side ; but it differs from its homologue in Ophiurids in that its 
cavity is already connected with that of the left anterior en- 
terocoel. Whether it is derived from this anterior enterocoel 
or from the posterior one, as is stated by Gotte (11, p. 609), 
Ludwig (17, p. 141), and Selenka (26, p. 49), is a question 
which, in spite of much time spent upon it, I am unable to 
answer satisfactorily. The latter is suggested by the analogy 
of the Ophiurids, but on the whole my observations seem to 
support the former, and to indicate that the communication 
above mentioned between the anterior enterocoel and hydrocoel 
VOL. XXIX, PART 4. NEW SER. 
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