210 



GASTKOPODA. 



merit of the pericardium ; the process is therefore very similar to 

 that described (p. 74) in connection with the Lamellibranchs. The 

 lumen of the sacs is to be regarded, here also, as further development 

 shows, as the secondary body-cavity, while the definitive body-cavity 

 proceeds from the cleavage-cavity which contains numerous scattered 

 mesoderm-cells. 



The whole mesoderm-rudiment is not, as already mentioned, used 

 up in the formation of the coelomic sacs ; occasionally even com- 

 pact masses of mesoderm remain which have been distinguished as 



a. 



-c. 



FIG. 



. Diagrammatic representations of young embryos of Bythinia tentaculata ; 

 A, frontal section ; B and C, from the right side (after v. ERLANGER). a, anal 

 region ; bl, blastopore ; c, coelom ; ent, entoderm ; m, mouth ; mes, mesoderm- 

 rudiment ; sd, shell-gland ; t, ectodermal thickening, from which the tentacles and 

 the cerebral ganglion are produced ; v, velum. 



cephalic or trunk-mesoderm and from which, by delamination, somatic 

 and splanchnic layers have been derived. According to this view, 

 the definitive body-cavity would arise at least to some extent in 

 the form of a coelom. This subject will be referred to again later 

 (p. 217). It is generally assumed that the definitive body-cavity 

 arises out of the primary body-cavity in which the cells detached 

 from the mesoderm-bands become distributed, yielding connective 

 tissue and musculature. With regard to the latter, the origin of 

 the columellar muscle has been somewhat more carefully examined ; 



