1 68 Introduction to Animal Morphology. 



3 per cent, of salts of soda, and a trace of albumen), 

 whose wall divides into a surface cuticle, and a deeper 

 muscular layer of circular (outside) and longitudinal 

 (inside) fibres, within which is a layer of nucleated 

 cells. At the end where the hooks were in the pro- 

 scolex, a swell ing of the sub-muscular cells takes place, 

 projecting at first equally on the outside and into the 

 saccular body. This grows inward, and divides into 

 two layers, an outer, receptaculum scolicis, and an 

 inner, proper body-wall of the scolex. The latter 

 grows, becomes worm-like, with a dilated, saccular 

 free end often coiled within the former, which becomes 

 flask-like and dilated. The water-vascular system 

 and its concretions early appear in this embryo, and 

 at the \vid-st end of its cavity the suckers and hooks 

 form. This scolex was long thought to be a distinct 

 (cystic) worm, until the researches of Kitrhcumcistcr^ 

 I'an llau-diii, tvc., showed it only to be a temporary, 

 immature condition. Cysts of this kind may give rise 

 to colonies (when they are called polycampic) either 

 by the growth of buds from the primary larval sac 

 (Ccenurus), or by the formation within the primary sac 

 of a secondary or daughter cell, which develops new 

 cells by peripheric budding (Echinococcus), or each 

 cv^i may only produce a single head (monocampic). 

 These cysts are common in the solid tissues of animals 

 (hydatids, "measles" in pork), and when introduced 

 into the stomachs of other animals, the outer cyst and 

 tail vesicle disappear, being dissolved away, the 

 suckers and hooks develop, and the worm moves in 

 the intestine of its host, sometimes carrying its caudal 

 vesicle with it. Hitherto the young worm has been 

 hollow, but parenchyma rapidly develops after the 



