412 
H. BURY. 
its way towards the anterior end of the larva, immediately 
ventral to the left anterior enterocoel. Prom the latter it is 
distinguishable by its more regular outline, the greater dis- 
tinctness of its lumen, and the more refringent character of 
its walls ; indeed, it is altogether so much more conspicuous 
an object than the overlying enterocoel, that it is easy to over- 
look the latter altogether, or to mistake it for a solid mass of 
mesoderm cells — an error into which previous observers appear 
to have fallen. 
A section, through a larva in this stage, just behind the 
water- pore, is given in fig. 19, and confirms the observations 
made upon the living animal. It is true that in this section 
the lumen of the right anterior enterocoel is not seen ; but 
this, when the small size of the cavity is considered, is not 
surprising. I have abundant evidence that the cavity still 
exists. 
Echinids. — The first stage which concerns us here is that 
of the young Pluteus with two completely separated enterocoels 
lying beside the oesophagus ; a satisfactory figure of this stage 
is given by Prouho (24, pi. xxiv, fig. 3). Whether these two 
cavities are separate from the first (24, p. 234), or whether they 
are at first united (26, p. 49), is a question which need not 
detain us now. Even at an early period the bilateral sym- 
metry is rendered incomplete by the development of a ciliated 
pore (water-pore) at the posterior end of the left enterocoel. 
The next stage in development is marked by the division of 
each of the primary enterocoel pouches into two lobes, one of 
which remains beside the oesophagus, while the other extends 
back to the side of the stomach. This was the latest stage ob- 
served by Prouho (24), but in all my larvae (belonging 
to Echinus microtuberculatus, Strongylocentrotus 
lividus, Spliaerechinus granularis, Echinocardium 
cordatum, and other unknown forms) each cavity soon 
divided completely into two, so that a stage was reached such 
as MetschnikofF (18 and 19) has already described, in which 
there exist two anterior and two posterior enterocoels, the only 
difference between my account and MetschnikofF’ s being that 
