Miscellaneous. 413 
dorsally convex, ventrally concave. The shell-gland now appears 
upon the dorsal surface of the hinder end as a thickening of the 
ectoderm, and simultaneously the rudiments of the cerebral ganglia 
arise as lateral thickenings of the velar area. At this stage there 
further arises the glandular portion of the primitive kidney as a 
little heap of mesoderm cells. At the hinder end of the blastoporal 
groove a little pit is observable, which marks the spot at which 
subsequently the contracted end of the archenteron breaks through 
the ectoderm to form the anus. 
The cesophagus arises by an invagination of the ectoderm at the 
spot where the mouth had originated from the blastopore, and 
exhibits in front of the mouth two large clear cells which belong to 
the velum. The velum itself is distinguished by the very large size 
of its cells, which show the concretions described by Sarasin, and 
are ciliated. It extends a very long way backwards. 
Soon after this the foot is formed as a protuberance of the ecto- 
derm on the ventral side behind the mouth. The cesophagus itself 
already exhibits the evagination of the radula pouch. Shell-gland 
and cerebral plates continue to increase in size, and the mesoderm 
grows round the archenteron more and more in a dorsal direction, 
while ventrally it gives rise to a considerable mass of cells, which is 
the rudiment of the pericardium. 
The primitive kidney is brought into communication with the 
exterior by means of an ectodermal excretory duct lying beneath 
the hump-shaped lateral projections of the velum. 
The embryo now grows more lengthwise, and its anterior end is 
distinctly marked off from the posterior, which bears the shell-gland, 
since it is separated from it by the foot. The kidney arises on the 
right side from a thickening of the pericardium; the latter has 
moved more towards the right and in a dorsal direction, owing to 
the torsion which now comes into play. The mantle-ridge appears 
somewhat later, and simultaneously there is formed in its vicinity a 
small invagination of the ectoderm, the rudiment of the excretory 
duct of the kidney. 
In the rudiment of the pericardium, which was hitherto solid, a 
lumen arises, the pericardial cavity ; the same thing happens in the 
kidney ; the two lumina come into connexion with one another by a 
narrow opening, while the kidney itself opens by its excretory duct 
into the mantle-cavity, which has arisen through the outward growth 
of the edge of the mantle. The heart is formed as an invagination 
of the wall of the pericardium ; it becomes constricted in the middle, 
and is thus divided into the auricle, which is situated in front, and 
the ventricle, which lies behind. 
The ganglia arise in precisely the same way as in Paludina™*, as 
separate thickenings of the ectoderm, which sever themselves from 
their place of origin, sink inwards, and then, and not before, come 
into connexion with one another by means of commissures and con- 
nectives. There is nothing to be seen of a continuous ingrowth of 
* R. v. Erlanger, “ Zur Entwicklung von Paludina vivipara,” Zool. 
Anzeiger, no, 357 (1891). 
Ann. & Mag. N. Hist. Ser. 6. Vol. ix. 31 
