DEVELOPMENT OF THE NEMATODES 



373 



The same manner of development occurs in other species of the same 

 genus-, and also in the case of Strongyloides stercoralis. 



(2) With Intermediate Host, 

 (a) Frequently, however, the 

 larvae of Nematodes make use 

 of one or even two intermediate 

 hosts ; their condition then 

 resembles that of Cestodes or 

 Trematodes, excepting that 

 there is never a multiplication 

 within the intermediate hosts. 

 The larvae become encapsuled 

 amongst the tissues of the in- 

 termediate host, and wait till 

 they are introduced with the 

 latter into the final host. For 

 instance, Ollulanus tricuspis, the 

 adult form of which is found in 

 cats, previously lives encysted 

 in the muscular system of mice. 



FIG. 269. A piece of the trunk muscle of the 

 pig with encapsuled embryonic Trichinae. Mag- 

 nified. 



Cucullanus elegans, which at- 

 tains the adult stage in fishes 

 (perch, etc.), is found encysted 

 in species of Cyclops. Other 



examples of species that require an intermediate host are Filaria 

 bancrofti and Dracunculus medinensis. 



Peculiar conditions prevail in the case of (6) Trichinella spiralis. 

 This species, which in its adult state lives in the intestine of man and 

 of various mammals, is viviparous ; the young Trichinae, however, do 

 not leave the intestine, but reach the intestinal \vall (Cerfontaine, 

 Askanazy) in the following way : the female intestinal Trichinae bore 

 into the intestinal wall, where they are found in the submucosa, or 

 in the lumen of the dilated lacteal vessels. Here the young are born, 

 in the intestinal wall, and leave this position with the lymph stream. 

 Some of them, no doubt, actively bore through the intestinal wall, 

 reaching the lymph or blood-stream, or even pass into the body 

 cavity. What occurs during their further migrations is difficult to 

 say at present. It has hitherto been maintained that the wandering 

 is entirely active ; for instance, the ligaturing of an artery would be 

 no protection against the part of the body supplied by such artery 

 being invaded by Trichinella. This observation cannot be otherwise 

 explained than by the active progress of the young Trichinella. The 

 question, however, may be mooted as to where and when the worms 

 quit the blood-vessels, which they for the most part reach through 



