40 Moll. 
MOLLUSCA. 
weeks old. The larva gradually becomes still more asymmetrical, and 
the right side of the shell develops a notch, which deepens and gives the 
animal the appearance of an asymmetrical Emarginula. The edges of 
the notch gradually unite, leaving an aperture, and the condition 
approaches that of Bimula , and finally the spiral body reaches the summit 
of the shell, and the aperture disappears ; Boutan (46). 
The larva of Gadinia passes through trochosphere and veliger stages, 
and the embryonic shell is spiral, not patelloid, as in Ancylus ; Lacaze- 
Duthiers (222a). 
Salensky (319) finds that in Vermetus even the unsegmented ova ex¬ 
hibit a differentiation into proto- and deuto-plasmatic portions ; directive 
corpuscles were not observed. Segmentation resembles that of other 
Mollusks ; after the cleavage into sixteen, the micromeres flatten out 
over the surface of the yolk, and after they have surrounded about two- 
thirds of it, the egg flattens and bends towards the ventral side, forming 
the archenteron. A round blastopore is situated in the centre of this 
surface, but gradually passes backwards and becomes oval. Small macro- 
meres are now formed by division, and form the secondary or definitive 
entoderm, which appears as a heap of small polygonal cells, filling the 
gastrula cavity, and projecting from the blastopore. The blastopore does 
not close, but becomes the mouth. The mesoderm arises from the ecto¬ 
derm around the blastopore ; at the same time appear the rudiments of 
the foot and the velum. The cephalic and pedal ganglia are independ¬ 
ently developed ; the former from two ectodermal plates, lying in front 
of the velum ; they become invaginated, and the posterior angles of the 
depressions become the points of origin of the eyes ; the ganglia remain 
a long time hollow; the auditory vesicles arise long before the pedal 
ganglia, as invaginations on the margins of the foot. The homology is 
suggested of the cephalic ganglia of Vermetus with those of Annelids, and 
of the pedal with the first ventral ganglia. The foot has two large 
glands, one already described by Lacaze-Duthiers, the other on the 
anterior margin, consisting of a compact mass of cells with a long duct. 
The mesoderm, at first of one layer, afterwards forms several, and splits 
into a somatopleure and splanchopleure, with a wide coelom between 
them; a posterior undivided portion becomes the musculus columellaris ; 
the pericardium appears early in the right side of the embryo, and forms 
a cavity homologous with the coelom. The heart is developed from the 
splanchopleure in the posterior angle of the pericardial cavity. The 
mid-gut is only formed after the escape of the larva ; the hind-gut 
earlier, as a plate of cylindrical entoderm cells. The oesophagus and 
radular pouch arise from the ectoderm surrounding the blastopore. 
Ziegler (383) gives an elaborate account of the development of Sphce- 
rium corneum, L. The egg has a vitelline membrane, and the segmenta¬ 
tion proceeds on the same type as that of Teredo (Hatschek) and Unio 
(Rabl). The blastula consists of a single layer of cells. Those forming 
the under surface show traces of an invagination, and are derived from 
the large yolk-containing cell ; the hinder part is indicated by certain 
deeply-stained cubical cells, which afterwards form the shell-gland ; 
within are one or two primitive mesoderm cells, probably derived from 
