208 THE ANIMAL PARASITES OF MAN 



reach the intestine of cats, those of the hare and rabbit (Cysti- 

 cercus pisiformis) reach the intestine of hounds ; those of the pig 

 (Cysticercus cellulose) are introduced into man, those of insects are 

 swallowed by insectivorous birds, those of crustaceans are ingested 

 by ducks and other water fowl ; perhaps, also, the infection of 

 herbivorous mammals is caused by their having accidentally 

 swallowed smaller creatures invaded by cysticerci. Of course, the 

 researches of Grassi and Rovelli have taught us that such an 

 intermediary host is not always necessary ; the Tcenia murina of 

 rats and mice in its cysticercus stage lives in the intestinal wall 

 of these rodents, and as a cysticercus it breaks into the intestinal 

 lumen and develops into a tapeworm in exactly the same way 

 as the cysticerci of other species that reach the intestine of the 

 terminal host by means of an intermediary carrier. Probably 

 the curtailed manner of transmission also occurs in rnariy other 

 species. In some cases the cysticerci quit the body of the inter- 

 mediary host actively as Ligula and Schistocephalus, which travel 

 out of the body cavity of infected fishes and reach the water, 

 where they may be observed in hundreds in summer, at all events 

 in some localities. The cysticercus stage of Calliobothrium wrongly 

 termed scolex has been observed swimming free in the sea, and 

 the scolices of Rhynchobothrium, without their parent cysts, have been 

 observed free within the tissues of some marine animals. Tn any 

 case there is almost always a change of hosts even in the single- 

 jointed cestodes, for the cysticercus of Caryophyll&us, which lives in 

 fishes of the carp family, is found in Oligochreta, that of Gyrocotyle 

 (chimaera) in shell-fish (Mactra) and different conditions can hardly 

 be possible for Amphilina. Archigetes alone becomes sexually mature 

 in the cysticercus stage, but the life-history of this creature is not 

 well known, so that it is not impossible that the attainment of sexual 

 maturity as a cysticercus in an invertebrate animal (Oligochseta) 

 is perhaps abnormal, and somewhat analogous to the maturity 

 of some encysted trematodes. 



The METAMORPHOSIS OF THE CYSTICERCUS into the tapeworm is 

 rarely accomplished in a simple manner ; the transformation, how- 

 ever, is not complex in the single-jointed cestodes, nor in Ligula 

 and Schistocephalus ; the latter is swallowed by birds (Mergus, 

 Anas, &c.), produces eggs after only a few days, and very soon 

 quits the intestine of its terminal host. In all other cases it is 

 the scolex only which, by proliferating at its posterior extremity, 

 forms* the proglottides, after having invaded as a cysticercus 

 the intestine of a suitable host. All parts belonging to such 



