PHYLUM PLATYHELMINTHBS 245 



(Lymncvus), as a parasite of which it is alone able to enter upon the 

 next phase in its life-history. When it meets with the Snail, 

 the embryo bores into it by means of the head-lobe, coming to 

 rest in the pulmonary sac or some other organ of the mollusc. 

 Established in the interior of the Snail, it loses its ectoderm and 

 grows rapidly into the form of an elongated sac, the sporocyst (Fig. 

 190, E), with an internal cavity containing germ-cells and lined 

 by a layer of cells, with remnants of the eye-spots, and with 

 flame-cells. The sporocyst may divide into two similar bodies by 

 a process of transverse fission, but this is exceptional. Eventually 

 cells are budded off from the layer that lines the internal 

 cavity of the sporocyst or from the germ-cells, and these 

 undergo a process of segmentation similar to the holoblastic 

 segmentation of the impregnated ovum, resulting in the 

 formation of a morula, which becomes converted into a stage 

 resembling a gastrula. The gastrula elongates and gives rise 

 to a body called a redia (C), which begins to move about, and, 

 eventually forcing its way out of the interior of the sporocyst, 

 finds its way to some other part of the Snail, usually the liver. 

 When fully formed, the redia is a cylindrical body with a pair 

 of short processes {proc.) near the posterior end, and with a 

 circular ridge near the anterior end. It possesses a mouth 

 leading to a pharynx and simple sac-like intestine, and there 

 is a system of excretory vessels. In the interior of the redia 

 cells are budded off and develop into gastrulge, exactly as in 

 the case of the sporocyst; these gastrulse either develop into 

 a fresh generation of redise if the season should be winter, or, if it 

 should be summer, give rise to bodies termed cercaria: The latter 

 (D) are provided with long tails : they have anterior and posterior 

 suckers, and a mouth and pharynx, followed by a bifid intestine. 

 An opening, the birth-opening (C, b. op.), is formed in the wall 

 of the redia near the circular ridge, and through this the cercarias 

 escape ; they move actively by means of their tails, and force their 

 way out of the body of the Snail. They then, losing the tail, 

 become encysted, attached to blades of grass or leaves of other 

 herbage. The transference of the larval Fluke in this stage to its 

 final host, the Sheep, is effected if the latter swallow the grass on 

 which the cercaria has become encysted. The young Fluke then 

 escapes from the cyst and forces its way up the bile-ducts to the 

 liver, in which it rapidly grows, and, developing reproductive 

 organs, attains the adult condition. 



iii. The Common Tape- Worm of Man (Tcenia solium). 



General Features. — Tcenia solium occurs as a parasite in the 

 intestine of man. It has the form of a narrow ribbon (Fig. 191), 

 which may attain a length of several yards, attached at one end to 



