162 Dr. E. Korschelt on the 
cavity takes place, and with this the embryo which has 
arrived at the Gastrula-stage assumes a roundish oval shape. 
The rudiment of the intestine arises by the growing inwards 
from in front of an ectodermal invagination towards the endo- 
dermal portion (the subsequent mid-gut) ; this is the fore- 
eut, which afterwards unites with the endoderm. The hind- 
gut appears to arise in a similar way, though it was impossible 
to decide this with certainty. I have likewise been hitherto 
unable to determine satisfactorily the origin and further 
differentiation of the mesoderm. At an early period meso- 
dermic cells may already be observed in the primary body- 
cavity, scattered about or collected together in groups, yet a 
doubt must be allowed to exist as to their relation to the 
mesodermic bands, which have been described for other 
mussels, and as to whether these bands are present in so 
definite a form. 
Before the development of the intestine has yet proceeded 
so far as has been indicated above there has appeared, in the 
shape of an invagination of the ectoderm, the rudiment of 
another organ of extreme importance for the development of 
the Lamellibranchs ; this is the shell-gland. ‘The embryo, or 
more correctly speaking the larva, assumes a broadly pear- 
shaped form. ‘The widening of the anterior portion is espe- 
cially pronounced: while even at an earlier period scattered 
cilia appeared at different parts of the body and caused it to 
rotate, they are now found packed more closely together on 
the thickened anterior portion, and here form a ciliated ring, 
which indicates the rudiment of the velum. This stage 
exhibits the greatest similarity to the marine larve (of Jodio- 
farta and Cardium) which are figured in Lovén’s paper *. 
We now have the Yrochophora-larva of Dretssena betore us, 
and it is an interesting point that this stage, which in the 
case of other freshwater mollusks (Cyclas, Pistdium, and 
Gastropods) is passed through in a more or less modified form 
within the egg-membrane, @. e. during the embryonic deve- 
lopment, is preserved as a free-swimming stage in the case of 
Dreissena. ‘The Annelids, too, which live in fresh water or 
upon the land (Oligocheetes and Hirudineans), pass through 
this stage, as is well known, only in a modified condition 
within the cocoon, so that free-swimming Trochophora-larve 
were not known in fresh water. 
Still essentially in the 7vochophora-stage is also the some- 
what older larva, in which the shell-membrane has con- 
* §. Lovén, “ Beitriige zur Kenntniss der Entwicklung der Mollusca 
acephala lamellibranchiata,’ aus den Abhandl. der k. Schwed. Akad. 
Wiss. fur das Jahr 1848 im Auszuge wbersetzt: Stockholm, 1879, 
