298 LARVAL FORMS. 



The body cavity is lined by a peritoneum, and very soon distinct - 

 vessels, formed by folds of the peritoneum, become established. Of 

 these there are three trunks, two lateral and a median in the prse-oral 

 lobe (proboscis), and in the body a ventral trunk above the nerve 

 cord, and an intestinal trunk opening anteriorly into the ventral one. 

 The vessels appear to communicate with the body cavity. 



In the course of the above changes the two ciliated bands dis- 

 appear, the hinder one first. The cilia covering the general surface 

 become atrophied, with the exception of those on the ventral side of 

 the prse-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 cesophageal protuberance joined the epiblast at 

 a previous stage the mouth becomes established (fig. 162 C, ?/i), 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 '. 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 prse-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 (1) 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 differentia- 

 tion 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 proboscis of an adult 

 female, and lose their ciliated bands. Germinal 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 larvaa, into which the spermatic balls are dehisced. 

 Neither mouth nor anus is formed. The further changes have not 

 been followed out. 



The larval males make their way into the oesophagus of the 

 female, where they 110 doubt live for some time, and probably 



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

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

 elruinthes and Kotifera. 



