172 PARASITES OF THE DOMESTIC ANIMALS 



its booklets and commonly becomes smTOunded by a capsule formed by 

 proliferation of the connective tissue of the host, though this does not 

 occur in all of the larval forms. 



At this stage the larva, which is now a mere vesicle containing more 

 or less fluid and as yet without a head, is referred to as the acephalocyst 

 (bladder-cyst), from which there may, in certain forms (echinococcus), 

 develop multiple daughter cysts (Fig. 117). By a process of budding 

 from the germinal wall, the acephalocyst now develops a further stage, — 

 the cephalocyst (proscolex. Fig. 112, b and c), containing one (cysticercus, 

 Fig. 107) or more (coenurus. Fig. 114) heads which conform with the 

 scolex of the adult worm except that the larval head is invaginated. 



If the larva while still living at this stage is conveyed to the digestive 

 canal of a suitable host for the adult worm, the head is evaginated from 

 the vesicle (Fig. 112, c), becomes detached from it, and, passing to the 

 intestine, fixes upon the mucosa by means of its suckers, to which attach- 

 ment the crown of hooks contributes if this is present. By a process of 

 budding, the scolex now proliferates a series of segments, each to be- 

 come bisexually complete (Fig. 90) . 



Sexual maturity of the segments marks the stage of the adult worm 

 which, with its entire series, constitutes the chain, or, as it has been 

 called by most writers, the strobila, a term which, with that of proglottid 

 for segment, is discarded in this work. 



Tabular Review of Life History of T^nia Saginata 

 Adult Tapeworm in intestine of man 



I 

 Egg. — Expelled from intestine. 



1 

 Hexacanth. — Freed from egg in digestive tract when 



I ingested by ox. 



Acephalocvst. — ^In striated muscle of ox. 



I 

 Cephalocyst (Cysticercus) .^ Same. 



Scolex. — Attached to mucosa of intestine of man 



I after ingestion of cephalocyst. 



Adult Tapeworm in intestine of man. 



Parasitism. — The tapeworms afford an example of extreme para- 

 sitism. So far as known, their existence is wholly dependent upon 

 alternate cystic and adult hosts, their development exhibiting no free- 

 living stage. So advanced is their degeneracy that there is little of 

 organization remaining excepting the procreative, and this has acquired 



