; 



sn 



PHYLUM PLATYHELMINTHES 231 



.ail, it loses its ectoderm and grows rapidly into the form 

 of an elongated sac, the sporocyst (Fig. 179, B), with an in- 

 ternal cavity, 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, 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 redid (C), which begins to move about, 

 and eventually forces its way out of the interior of the sporocyst. 

 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 (C). 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 gastrulse, exactly as in 

 the case of the sporocyst ; these gastrulse either develop into 

 a fresh generation of redise, or give rise to bodies termed 

 cercarice. 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, birth-opening (b. op.), 

 is formed in the wall of the redia near the circular ridge, and 

 through this the cercariue 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 soliuni]. 



General Features. Tcenia solium occurs as a parasite in the 

 intestine of man. It has the form of a narrow ribbon (Fig. 180), 

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

 the wall of the intestine, the remainder hanging freely in the 

 interior. Towards the attached end the ribbon becomes very much 

 narrower than it is towards the opposite end ; and at this narrower 

 extremity is a small, rounded, terminal knob, which is known as the 

 .head or scolex ; the rest of the animal is termed the body or 

 strobila ; the narrow part immediately behind the head is some- 

 times termed the neck. The attachment of the Tape-worm to the 



