166 



PEACTICAL PABASITOLOGY 



size of these hooks have a certain specific value. In many varieties 

 a narrow neck-piece is interposed between the proboscis and the 

 trunk, and the latter frequently bears a varying number of bristles. 



The body-wall is comparatively thick. 

 Through it, either in patches or extending 

 over the entire body, a network of lighter 

 channels can be seen. This network 

 generally proceeds from two main stems, 

 !' \Hil an< ^ ^ as no a PP aren ^ communication with 



the exterior. It runs from the proboscis, 

 through the neck into the trunk, where it 

 is continued into two appendages of the 

 body-wall, known as the lemnisci. These 

 are saclike structures, dissimilar in size 

 and shape, which project into the body- 

 cavity. A fluid circulates in the canal 

 system, derived from the contents of the 

 intestine ; it contains a varying proportion 

 of granules. 



Within the anterior portion of the body- 

 cavity, situated in the axial line, is a long 

 sac-shaped organ with thick muscular walls ; 

 this is the proboscis sheath. Its function 

 is twofold ; it acts as a receptacle for the 

 proboscis when withdrawn, and the extru- 

 sion and evagination of the proboscis is 

 effected by means of the muscular con- 

 tractions of its walls. The proboscis is 

 retracted by means of a longitudinal muscle 

 running in the axis of the sheath, one end 

 of which is attached to the body-wall, 

 while the other end is attached to the inner 

 surface of the distal end of the proboscis. 

 A second longitudinal muscle is attached 

 to the base of the sheath, and runs dia- 

 gonally across the body-cavity in a back- 

 ward direction, to a point of attachment 

 in the body-wall (retractor receptaculi) 

 (fig. 85). 



In those species in which the walls of 

 the proboscis sheath are thin, a rounded 

 mass, of cells will be made out near its 

 posterior extremity. This is the nerve 

 ganglion and from it the nerve-cords, usually 



FIG. 85. Echinorhynchus 

 augustatus. Male, 25 : 1. Of 

 the three organs placed imme- 

 diately behind the proboscis, 

 the central one is the pro- 

 boscis sheath, those on either 

 side the lemnisci. The re- 

 tractor of the proboscis sheath 

 takes its rise at the base of 

 the sheath. By it, to the right, 

 are the oval testes ; behind 

 them, the darker cement- 

 glands; and behind these again, 

 the penis. (After Leuckart.) 



