BIOLOGY AND STRUCTURE OP' HYDROPHILUS CARABOIDES 639 



Farther backwards, the ahmentary tube when compressed 

 takes the shape of a groove, the upper edges of which are tied 

 together by muscular bundles ; the levators running in an 

 oblique direction are located at the bottom of the groove, 

 and between them, just in the middle, there are muscular 

 bundles which, perhaps, serve for the contraction of the 

 part of the intestine here described. 



Nearer to the level of the brain-ganglia (Text-fig. 11, sjje, 

 soe) the structure of the intestine is different. The walls of the 

 tube grow thinner. In section it looks like a strongly plicated 

 ring surrounded by circular muscular filaments (second 

 sphincter, Text-figs. 11, 12, So). They alternate with bundles 

 of the radiating dilatators of the middle part of the fore-gut. 

 There are four groups of dilatators — two upper (rZZg) ^^^^ two 

 lower ones ; the upper groups are fastened to the upper wall 

 of the head, the under ones go round the sub-oesophageal 

 ganglion and, as it appears, partly terminate on the chitinous 

 fold of the endoskeleton which lies above the fore end of the sub- 

 oesophageal nervous ganglion. Behind the pharyngeal ring the 

 intestine retains its form of a thin- walled tube strongly plicated 

 longitudinally, clad in the usual muscular covering. The fore- 

 gut opens directly into the mid-gut and does not form any 

 valve or cardiac fold at the border of the stomach. 



On the whole it may be said that the fore-gut of the larva of 

 H. caraboides is provided with two muscular apparatuses : 

 (1) the suctorial apparatus, which sets into action the first group 

 of sphincters and levators (Text-fig. 10), and (2) the pharyngeal 

 apparatus (the second suctorial apparatus), the sphincter and 

 dilatator of which act on to the middle part of the fore-gut 

 (Text-fig. 11). 



Having acquainted ourselves with the structure of the fore- 

 gut of the larva of H. caraboides we can 'picture its 

 functions as follows. 



With the contraction of the second group of sphincters 

 (Text-fig. 11, s^) the middle portion of the fore-gut is tightly 

 closed. Then the levators of the suctorial apparatus (Text- 

 fig. 10, dl^) contract and lift up the upper wall of the digestive 



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