138 GASTROPODA. 



dorsal surface by the growth of its free edge which is still in close 

 contact with a thickened layer of ectoderm, the cells of this 

 thickening being concerned in the secretion of the shell. Beyond 

 this thickening, the mantle-fold (Fig. 58, mf), as a slight upgrowth 

 of the ectoderm, is situated dorsally to the anus. When the latter is 

 displaced forward by the more rapid growth of the posterior dorsal 

 part of the body (Fig. 59 ^4), the mantle either grows out further or 

 else the surface of the body behind the mantle and in front of the 

 anus sinks in somewhat ; the depression which is thus formed is the 

 rudiment of the mantle-(pallial) or branchial cavity (Fig. 59 A, ink). 

 The anal aperture now comes to lie in this depression. 



Turning to the internal organs, we find that the fusion of the 

 stomodaeum with the enteron has now taken place (Fig. 58). The 

 rudiment of the liver appears ventrally as a sac-like outgrowth 

 of the enteron, and the radular sac arises from the stomodaeum. 

 As the mesodermal structures (the pericardium, the heart and the 

 kidneys, Fig. 59) become differentiated, those on the right side 

 attain a greater size than those on the left, so that a marked asym- 

 metry is already evident in these internal organs. The rectum, which 

 formerly ran directly backward, now comes to lie at right angles to 

 the longitudinal axis in consequence of the displacement of the anus 

 described above and, later, runs obliquely to the right side. 



The inner asymmetry precedes the outer, and has therefore been 

 used as an explanation of the asymmetrical structure of the body 

 (Bythinia, P. SARASIN, No. 101). When we spoke above of a dis- 

 placement of the anus the expression used was not strictly accurate, 

 since the distance between the mouth and the anus remains almost 

 the same. Marked growth, on the contrary, takes place first in the 

 dorsal surface and later especially in the left posterior part of the body. 

 Although the area lying between the mouth and the anus does not 

 grow appreciably, considerable increase in size occurs in the posterior 

 region (Fig. 59 A-C), and it results that the parts that have not grown 

 now seem to belong more to the anterior portion of the body which 

 as a whole is now much larger. BUTSCHLI has paid special attention 

 to these processes in Paludina (No. 19). The left posterior part of 

 the body, in consequence of the processes of growth just described, is 

 much swollen and this leads to the formation of the visceral sac 

 directed backward to the left and to the (apparent) shifting forward 

 to the right of the anus and the parts surrounding it. The swelling 

 of the posterior dorsal parts of the body to form the visceral sac is 

 determined by the advancing growth of the inner organs. The 



