178 GASTROPODA. 



exceedingly large archenteron (md). Some of the cells of the latter, in 

 consequence of the albuminous matter which has been brought from 

 without through the mouth into the lumen of the intestine, have a 

 swollen appearance (Figs. 70-80) ; others, however, which lie pos- 

 teriorly and ventrally are smaller, indeed, through more active 

 division, they may even be specially small. They form a diverti- 

 culum of the entoderm which is directed backward (Fig. 78) and 

 represent the rudiment of by far the greater part of the enteron. 

 The cells containing albumen, which continue to increase in size, pass 

 over into the formation of the liver later. At first the intestinal 

 cavity appears bounded partly by a large-celled and partly by a 

 small-celled epithelium. The posterior diverticulum of the enteron 

 comes into contact with the ectoderm in the ventral middle line, 

 behind the foot. This part at first bulges somewhat outward, form- 

 ing the anal prominence. Later on, the entoderm-diverticulum here 

 fuses with the ectoderm to form the anus. 



The resemblance of the stage just described to the Trochophore 

 stage is heightened by the presence of a paired primitive kidney, 

 which, in the fresh- water Pulmonates, has a very characteristic 

 origin and shape (Figs. 78-80, un). Even at an early stage, a 

 remarkably large cell can be seen on each side below the dorsal part 

 of the velum ; these two cells yield the principal constituents of the 

 primitive kidney, and have been claimed as velar cells which have 

 entered the body-cavity (WOLFSON), this view being no doubt sug- 

 gested by the vacuolated character of the velar cells as well as by 

 the condition of those Prosobranchs in which complexes of ecto- 

 dermal cells which are certainly excretory are apparently closely 

 related to the velum. This view can, however, hardly be correct, 

 and, taking into consideration the usual method of formation of the 

 primitive kidneys, we prefer the view of EABL that these large cells 

 are to be derived from the mesoderm.* They lie at the posterior 

 part of the mesoderm-bands which are already disintegrating. In 

 each of these cells, a cavity which at first resembles a vacuole ap- 

 pears, lengthening as soon as the cell itself lengthens. The cell then 

 becomes bent and forms the principal part of the primitive kidney, 

 the canal of which is thus intra-cellular in its origin (GANIN, No. 35 ; 

 EABL, No. 91 ; WOLFSON, No. 131). The large cell yielding the 

 primitive kidney is joined by a few of the adjacent mesoderm-cells 

 and the canal, by becoming connected with the ectoderm, opens 



* [See footnote, p. 179. ED.] 



