Miscellaneous. 413 



dorsally convex, vcntrally concave. The shell-gland now ap])Cais 

 tipon the dorsal surface of the hinder end as a thickening ot 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 

 subseciuently the contracted end of the archenteron breaks through 

 the ectoderm to form the anus. 



The (esophagus 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 distir.guished 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 oesophagus itself 

 already exhibits tlie cvagination 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 tlie 

 right side from a thickening of the pericardium ; the latter has 

 moved more towards the right and in a dorsal direction, owing to 

 tlie 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 luraina 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. cO Mag. N. Hist. Ser. 6. VoL ix. 31 



