44 



AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



and they are made up of exceedingly delicate muscle-cells which 

 are of considerable length and taper at both ends. 



The nervous system of a tapeworm consists of two longitudinal 

 cords, one of which traverses each side of the body near its 

 margin (Fig. 10), and of a transverse brain-commissure by which 

 these two cords are connected in the head. Nerves run from the 

 commissure to the hooks, suckers, and other parts of the scolex. 

 Histologically the nervous system is essentially made up of nerve- 

 cells and nerve-fibres, the former of which are not limited to the 

 cerebral commissure. 



DEVELOPMENT. 



The very numerous eggs pass into the uterus, which, at first a 

 simple tube, becomes much branched (Fig. 9), and fills up the 

 greater part of the ripe proglottis, while at the same time the 

 remaining sexual organs gradually abort. 



The minute oosperm first undergoes 

 complete cleavage, and then is gradu- 

 ally converted into a spherical six- 

 hooked embryo (Fig. 11), in the devel- 

 opment of which the yolk-cells enclosed 

 i in the e SS are gradually used up. 



The ripe proglottides pass out of 

 the body of the host, and gradually 

 decay, the innumerable embryos con- 

 tained being thus liberated. If any 

 of these are now swallowed by a pig 

 (or other warm-blooded animal) the egg 

 shells are dissolved, and the embryos, 

 by means of their hooks, penetrate the 

 walls of the intestine and pass into 

 blood-vessels, being then carried in the 

 circulation to the muscles. In these they pass into the cysticercus 

 or bladder worm stage (Fig. 12), consisting of a fluid-filled vesicle, 

 (proscolex) into which a tapeworm-head (scolex) projects as a 

 hollow bud, the whole being surrounded by a firm cyst developed 

 from the surrounding tissue as a result of irritation. Develop- 

 ment proceeds no further in the pig, but if pork infested by these 

 cysts (" measly " pork) is taken into the stomach of a human 



d 



Fig. 11. TAPEWORM (from 

 Landois and Stirling). 

 Ripe egg from uterus of 

 Tsenia solium : a, albumin- 

 ous envelope; 6, remains 

 of yolk ; c, covering of 

 embryo ; d. embryo with 

 booklets. 



