186 



GASTROPODA. 



enteron can be seen the tube of the primitive kidney which is as yet 

 unbent and which, according to Fol, opens outward at the posterior 

 base of the foot. Almost in this region, but somewhat behind the 

 foot, lies an organ described by Fol as the larval heart. 



The so-called larval heart (Fig. 82, Ih) consists of a bulging of the ectoderm 

 with which numerous mesoderm-cells become connected. This specially 

 differentiated part of the covering of the body which, when the mantle- 

 cavity forms later, is drawn into it and thus comes to lie more to the right, 

 carries on regular pulsations and is regarded by Fol as an organ for promoting 

 the embryonic circulation. It thus belongs to the category of larval hearts 

 which have already been alluded to (p. 152). 



While the cephalic vesicle in the later stages decreases in size, the 

 foot lengthens considerably. At first it is cylindrical, but it soon 

 spreads out more and more and now becomes a massive club-shaped 

 organ (Fig. 83), which is known as the caudal vesicle, and more 

 recently has been named the podocyst (Jourdain, Sarasin). As it is 

 richly supplied with mesoderm-cells which become applied to its wall, 

 it is capable of contraction and carries on rhythmical movements which 

 alternate with those of the cephalic vesicle. It is evident that this 

 large vesicular swelling is a circulatory or respiratory apparatus and 

 it may be that it also serves for nutrition, since diosmotic processes 

 take place in it. 



The podocyst is specially large in the embryos of various species of 

 Helix (Gegenbaur, v. Jhering, Fol, Sarasin). It here spreads 

 out laterally, and thus assumes the form of a broad plate which, 

 towards the end of the " larval period," lines the whole of the inner 



cavity of the egg-shell. P. 

 and F. Sarasin, in describing 

 a Helix {Acavus Waltoni, Fig. 

 Si) found in Ceylon, show that 

 the podocyst covers like a cap 



nutr. 



the shell of the very large 

 embryo in which several coils 

 have already developed. In this 

 form also, in which the pedal 

 vesicle is specially highly de- 

 veloped, pulsating movements 

 were perceived in that organ. 

 When it has reached its highest 

 development, two wide canals within the foot start from the vesicle, 

 one passing to the brain along the ventral side and the other running 



Fig. 84. — Embryo of Helix (Acavus) 

 Waltoni, seen from the side (after I'. and 

 F. Sarasin). Icb, cephalic vesicle; ml, 

 oral Lobes; «"'•, mantle-swelling (collar) ; 

 //</, podocysl ; s, shell ; spl, sensory plate ; 

 t, upper. /'. lower tentacle. 



