THE METAMORPHOSIS OF ECHINODERMS. 73 
facts shown in this figure by means of sections, one of which is 
shown in fig. 28. 
The anterior end of this mesentery soon breaks down, but 
before it does so the rudiments of the terminal plates appear 
(fig. 27) lying over the left body-cavity, though one of them, as 
the figure shows, is very much to the right of the intestine. 
This constitutes one of the most striking distinctions between 
the two types of Asterid larve (compare fig. 18); the other 
principal differences may be briefly summed up as follows: 
1. The larval cesophagus persists in Brachiolaria, and is not 
replaced by a new one, though it loses its functional activity 
for some time after the metamorphosis. 
2. Up tothe moment the metamorphosis begins, the hydrocce:l 
is still open to the anterior body-cavity, but this opening is not 
(for the most part, at least) in the same interradius as the 
water-tube and water-pore—one of the pouches of the hydroccel 
lying (see 9, pl. xxvi, fig. 22) anterior to the pore. 
The closing of the hydroccel ring I have not satisfactorily 
followed in Brachiolaria, but it is certainly in the interradius 
indicated by Ludwig (15, p. 169, and pl. vii, fig. 95), and 
probably in every way the same as in Asterina. 
3. As in Asterina, the most anterior hydrocel pouch of the 
larva does not unite with the most anterior of the dorsal 
arm-rudiments, but with the one just behind the anus—the 
whole hydrocel ring being rotated (viewing the animal from 
the oral surface of the adult—left side of the larva) in a 
direction opposite to that of the hands of a watch ; or in other 
words, the hydroceel is, so to speak, unscrewed slightly from 
the rest of the body. 
4, In the later stages of Bipinnaria asterigera the future 
oral surface is turned rather towards the posterior end of the 
larva; in Brachiolaria, however, it is the future aboral surface 
which is directed backwards (compare 24, pl. ii, figs. 1 and 5). 
The growth of the left body-cavity round the hydroceel, and 
the formation of an axial sinus between the two horns of this 
cavity, takes place as in Bipinnaria asterigera. 
The ‘dorsal sac” appears at a very early stage as a space 
