314 



INVERTEBRATA 



CHAP. 



:ur 



per 



of Paludina, agreement happily reigns among observers. The foot 

 appears as a mid- ventral protrusion, the shell gland as a mid-dorsal 

 shallow invaginatiou. Just as in the case of Patella, the shell gland 

 is everted aud converted into a shell-forming area covering a visceral 

 hump. The mantle fold aud mantle groove appear in the same way 



as in Patella, and the 

 A torsion process takes 



place apparently slowly, 

 as all stages in its com- 

 pletion are often found. 

 Before this happens, 

 however, the rudiments 

 of many organs appear. 

 To begin with, the com- 

 pact mass at the hinder 

 end of each mesodermic 

 band becomes hollowed 

 out, as we have already 

 seen, to form a small 

 pericardial vesicle (Fig. 

 247), and from each of 

 these vesicles an evagina- 

 tion, the rudiment of a 

 kidney, is found. The 

 two coelornic vesicles are 

 at first separated by a 

 septum, but this is soon 

 absorbed and a single 

 vesicle,the pericardium, 

 results (Fig. 249). This 

 lies ventral to the in- 

 testine near the posterior 

 end of the embryo. The 

 rudiment of the heart 

 appears as a dip in the 

 dorsal wall of this sac, and 

 in this way a bag full 

 of blastocoelic fluid is 

 formed which hangs down 

 into the pericardium and 

 constitutes the heart 

 with its contained blood. 

 The kidney on the right side becomes marked off from the 

 pericardium by a constriction, and this narrow communication forms 

 the reno-pericardial canal of the adult. On the left side the kidney 

 rudiment remains small and thick-walled, and is also marked off from 

 the pericardium by a constriction. 



The ureters or external sections of the kidneys arise as 



m.f 



liir Ik r.ur 



rrif. 



FIG. 249. Illustrating the development of the ureters of 

 1'nliKli mi riri/iitru and their relation to the kidneys. 

 (After Erlanger.) 



A, cut-off visceral hump of an embryo, rather older than 

 that represented in Fig. 24S, viewed from below. B, horizontal 

 section through the visceral hump of an embryo of the same 

 age as that represented in A. a.p, anal papilla; lifp, liver; 

 l.l;, left kidney; 1. in: left ureter; m.f, mantle fold; )ier, peri- 

 cardial sac (the right and left pericardial sacs of the earlier 

 stage have fused) ; )-./,-, right kidney ; r.ur, right ureter. 



