316 



INVERTEBRATA 



CHAP. 



After the embryo has escaped froai the womb of the mother, the 

 genital organs develop. According to Miss Drummond (1902), the 

 genital cells are budded from the pericardial wall, close to where 

 the original left kidney joins it, and this kidney forms the first part 

 of the genital duct. Coincidently with this development the peri- 



FlG. 250. — Two embryos of Paludina vivipara viewed from the light side in order to show 

 the origin of the sense-organs and the beginning of torsion. (After Erlanger.) 



A, stage when the foot and visceral hump are both short, and the embryo is almost bilaterally 

 symmetrical. B, stage when foot and visceral hump have become elongated, and in which torsion has 

 begun. The arrow show.s the direction in which torsion takes place, a, anus ; /, foot ; H, heart ; hep, 

 liver ; int, intestine ; l.-n, larval kidney ; l.p&T, left pericardial sac ; Z.wr, left ureter ; mx, mantle cavity ; 

 m./, mantle fold ; oc, eye ; oes, oesophagus ; ot, otocyst ; r.per, right pericardial sac ; r.$, radula sac ; 

 r.ur, right ureter ; sh, shell ; st, stomach ; ie% tentacles ; V, velum. 



cardium is shut off from the genital rudiment. The outer and longer 

 portion of the genital duct is formed by the left ureter (Fig. 252). 



The torsion of the visceral hump, which results in the transference 

 of the mantle cavity from the posterior aspect of the hump to its 

 anterior face, is to be carefully distinguished from the spiral twisting 

 of the hump, which is shown by the spirally twisted shell ; since the 



