XX 



LIFE-HISTORY 



493 



organs that assist in introducing the spermatozoa into the 

 female. The two tubes then unite, and having received a 

 dorsally-placed accessory gland, open to the exterior by a median 

 aperture placed ventrally a little way behind the mouth. 



Life - history. The egg undergoes a large portion of its 

 development within the body of the mother. In Linyuattilii 

 taenioides, which lives in the nasal cavities of the dog, the eggs 

 pass away with the nasal excretions. If these, scattered about 

 in the grass, etc., be eaten by a rabbit, the egg-shell is dissolved 

 in the stomach of the second host and a small larva is set free. 

 In Porocephalus proboscideus and others, which inhabit the lungs 

 of snakes, the eggs pass along the alimentary canal and leave the 

 body with the faeces. They also must be eaten by a second host 

 if development is to proceed. 



The larva which emerges when the egg-shell is dissolved has 

 a rounded body provided with two pairs of hooked appendages, 

 and a tail which is more 



or less prominent in 

 different species (Figs. 

 259, 260). Each ap- 

 pendage bears a claw, 

 and is strengthened by 

 a supporting rod or 

 skeleton. Anteriorly the 

 head bears a boring ap- 

 paratus of several chitin- 

 ous stylets. The various 

 internal organs are in 

 this stage already formed, 

 though in a somewhat 

 rudimentary state, and it 

 is doubtful if the anus 

 has yet appeared. 



By means of its boring 



FIG. 259. A late larval stage of /'/>>/,/,, >/".< ///</- 

 / videlis, seen from the side. Highly mainiilinl. 

 (From Stiles.) 1, primordium of lirst pair of 

 chitinous processes ; "1, priniordium of second 

 pair of ' liitmi'iis processes ; ','>, mouth : 1, vcnti:il 

 ganglion ; :"i, reeeptacuhini scininis ; (i, oviduct : 

 7, ovary ; N, anus ; !), vagina. 



apparatus, and aided by 

 its hooked limbs, the larva now works its way through the 

 stomach-walls of its second host, and comes to rest in the liver 

 or in some other viscus. Its presence, in the tissues of its second 

 host causes the formation of a cyst, and within this the larv;i 

 rests and develops. In man, at least, the cysts often undergo a 



