162 Dr. E. Kovsclielt on the 



cavity takes jjlace, and with this the einbiyo 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 raid-gut) ; this is the fore- 

 gut, 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 nieso- 

 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 develo])ment 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 jnonounced : 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 ])ortion, and here form a ciliated ring, 

 which indicates the rudiment of the velum. This stage 

 exhibits the greatest similarity to the marine larvte {oi Modio- 

 laria and Cardium) which are figured in Loven's paper *. 

 We now have the Troc/io2)Jiora-\iuya of Dreissena before us, 

 and it is an interesting |ioint that this stage, which in the 

 case of other iVcshwater mollusks {Ci/chm, Fisidiuin, and 

 Gastropods) is passed through in a more or less modified form 

 within the egg-membrane, i. 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 (Oligocha^tos and llirudineans), pass through 

 this stage, as is well known, only in a modified condition 

 wiiliiu the cocoon, so that free -swimming Trochophora-larvae 

 were not known in fresh water. 



Still essentially in the Troc/iopkora- stage is also' the some- 

 what older larva, in which the shell-membrane has con- 



* S. Lovt5n, " Beitiiige ziir Keimtniss der Entwickluu^ der Mollusca 

 ucepliala laiuellibninrhiata/" aiis deu Abhandl. der k. ScliwoJ. Akud. 

 Wis^s. I'ui- diis Jahr 1S48 im Auszu're uber!?elzt : Stockholm, isrit. 



