THE METAMORPHOSIS OF ECHINODERMS. 53 
entirely does it differ from anything which has come under 
my notice. We can hardly have obtained different larve, 
since we both worked at the same place (Naples) and at about 
the same time of year (January to the end of May, 1888, in my 
case; November, 1885, to October, 1886, in his—but few 
larvee found after March) ; and although during my subsequent 
visit to Naples in 1893 I met with many abnormal specimens 
(obviously pathological, as the condition of the whole ecto- 
derm showed) yet none of them ever approached those figured 
by Semon as intermediate between Auricularia and the pupa 
(82, pl. i, figs. 5 and 6). Stranger still is the fact that while 
these figures are wholly irreconcilable with Metschnikoff’s, he 
does not even allude, in this connection, to any differences 
between their accounts. 
* Hyprocawzt, &c.—On a previous occasion (5, p. 11) I 
described the formation of a cavity which I regarded as the 
homologue of the left anterior body-cavity in the Echino- 
derms. As Ludwig (17, p. 609) seems to think this homology 
disproved by his observations on Cucumaria, it will be well 
to review briefly the grounds on which my suggestion was 
based. 
When Ludwig first noticed this cavity (which he calls the 
“‘ Madreporenblase ”) in Cucumaria it had the appearance of 
a simple swelling on the water-tube; and as he could not 
find any trace of it on the previous (fourth) day, he concludes 
that it is altogether secondary. 
Whatever may be the case in Cucumaria, this conclusion is 
not justified in Auricularia. There, after the separation of 
the posterior body-cavities, the anterior portion of the coelom 
forms a small, pear-shaped, thin-walled vesicle, from which a 
short tube with thicker walls (“ pore-canal”’) runs to the 
exterior. Then the left wall of the vesicle thickens and 
presently produces the rudiments of the radial canals and 
tentacles; but the walls of the dorsal portion, into which the 
pore-canal opens, still remain thin. Shortly before metamor- 
phosis this thin-walled portion becomes constricted and divided 
into two, the smaller of which remains in connection with 
