DEVELOPMENT OF THE TAPEWORMS 305 



the intestine of cats ; those of the hare and rabbit (C. pisiformis) reach 

 the intestine of dogs; those of the pig (C. 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 

 accidentally swallowing smaller creatures infected by larvae. Indeed, 

 the researches of Grassi and Rovelli have taught us that such an 

 intermediate host is not always necessary ; Hymenolepis nmrina of 

 rats and mice in its larval stage lives in the intestinal wall of these 

 rodents, and as a larva it passes into the intestinal lumen and develops 

 into a tapeworm in exactly the same way as the larvae of other species 

 that reach the intestine of the terminal host by means of an inter- 

 mediate carrier. Probably this curtailed manner of transmission also 

 occurs in many other species. In some cases the larvae actively quit 

 the body of the intermediate host, as in the case of Lignla and Schisto- 

 cepJiahis, which travel out of the body cavity of infected fish and 

 reach the water, where they may be observed in hundreds in summer, 

 at all events in some localities. The larval stage of Calliobothriuin 

 wrongly termed Scolex has been observed swimming free in the sea, 

 and the scolices of Rhynchobothrimn, without their mother cysts, have 

 been observed free within the tissues of several marine animals. In 

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

 jointed Cestodes, for the larva of Caryophyllceus, which lives in fishes 

 of the carp family, is found in limicoline Oligochaetes, that of Gyro- 

 cotyle (Chimaera) in shell-fish (Mactra), and different conditions can 

 hardly be possible for Amphilina. Archigetes alone becomes sexually 

 mature in the larval 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 larva in invertebrates (Oligochaetes) is perhaps abnor- 

 mal, and somewhat analogous to the maturity of some encysted 

 Trematodes. 



The METAMORPHOSIS OF THE LARVA into the tapeworm is rarely 

 accomplished in a simple manner ; the transformation, however, is 

 not complex in the single-jointed Cestodes, nor in Ligula and Schisto- 

 cephalus ; the latter is swallowed by birds (Mergus,Anas, etc.), 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 proglottids, after 

 having invaded as a larva the intestine of a suitable host. The mother 

 cysts, or what corresponds to them, die, are digested, absorbed, or 

 perhaps even eliminated ; on the contrary, segments found on the 

 scolex during the larval stage, also in the case of Cysticercus fasciolaris, 

 are retained. It is not certain whether the larvae of Dibothriocephalus 

 lose any part. 



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