PHYLUM PLATYHELMINTHES 



243 



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. 194), 

 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 x ; the rest of the animal is termed the body or 

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

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

 wall of the intestine is slight and temporary ; it is effected by 

 certain organs of adhesion, the hooks and suckers on the head. 



The head (Fig. 195) may be roughly described as pear-shaped, 

 but becomes four-sided at the broader end. In the middle of this 

 broader, anterior end is a rounded prominence, 

 the rostellum, round the base of which there is 

 a double row of usually about twenty-eight 

 curved and pointed chitinous hooks. The 

 rostellum is capable of being protruded and 

 retracted to a slight extent, and the position 

 of the hooks varies accordingly : when the 

 rostellum is fully retracted the points of the 

 hooks are directed forwards, and may even 

 meet in the centre ; as the rostellum is pro- 

 truded the hooks become rotated until their 

 apices come to be directed backwards. Four 

 cup-shaped suckers project slightly from the 

 surface behind the circlet of hooks. 



The body or strobila has a jointed appear- FIG. 195. Head of 

 ance, owing to its being made up of a string fAfter^euckfu 

 of segments, or proglottides about 850 alto- 

 gether (cf. p. 510). These are narrower and shorter in front, gradually 

 increasing in size towards the posterior free extremity. The neck, 

 or part immediately following the head, is devoid of any trace 

 of segmentation. The two surfaces of the proglottides are not to 

 be distinguished by any differences visible to the unassisted eye ; 



1 Though very probable, it is not certain that this end of the Tape-Worm 

 actually corresponds to the anterior end iu the Liver-Fluke, as will be 

 explained later. 



R 2 



