360 LARVAL FORMS. 



disappear, the hinder one first. The cilia covering the general 

 surface become atrophied, with the exception of those on the 

 ventral side of the prae-oral lobe. The latter structure becomes 

 more prominent ; the stellate mesoblast cells, which fill up its 

 interior, become contractile, and it gives rise to the proboscis 

 (fig. 162 C). 



At the point where the oesophageal protuberance joined the epiblast at 

 a previous stage the mouth becomes established (fig. 162 C, ;/;), and though 

 it is formed subsequently to the atrophy of the anterior ciliated band, yet 

 there is evidence that it is potentially situated behind this band. The lumen 

 of the alimentary canal becomes established by the absorption of the 

 remains of the four central cells. The anus is formed on the ventral side 

 of the posterior end of the body, and close to it the pouches already noticed 

 grow out from the hindermost part of the alimentary tract (fig. 162 C, an.v). 

 They are at first simple blind pouches, but subsequently open into the 

 body cavity 1 . They become the anal pouches of the adult. There is present 

 when the mouth is first formed a peculiar process of the alimentary tract 

 projecting into the prae-oral lobe, which appears to atrophy shortly after- 

 wards. 



After the formation of the mouth, there are formed on the ventral side of 

 and slightly behind it (i) anteriorly a pair of tubes, which appear to be 

 provisional excretory organs and soon disappear (fig. 162 C, sc) ; and (2) 

 behind them a pair of bristles (s) which remain in the adult. The formation 

 of the permanent excretory (?) organ (oviduct and uterus) has not been 

 followed out. The ovary appears very early as a differentiation of the 

 epithelium lining the ventral vessel. 



The larvae, which become the minute parasitic males, undergo 

 a very different and far less complete metamorphosis than those 

 which become females. They attach themselves to the pro- 

 boscis of an adult female, and lose their ciliated bands. Germi- 

 nal cells make their appearance in the mesoblast, which form 

 spherical masses, and, like the germinal balls in the female 

 ovary, consist of a central cell, and an epithelium around it. 

 The central cell becomes very large, while the peripheral cells 

 give rise to the spermatozoa. A body cavity becomes developed 

 in the larvae, into which the spermatic balls are dehisced. 

 Neither mouth nor anus is formed. The further changes have 

 not been followed out. 



1 The fact that these pouches are outgrowths of the alimentary tract appears to 

 preclude the possibility of their being homologous with excretory tubes of the Platy- 

 elniinthes and Rotifera. 



