348 XEMATIIELMrSTHES. 



todes, have a different habitat to that of the sexual animal; the 

 young and the adult Nematode being contained in different organs 

 of the same or even of different animals. The larvae live for 

 the most part in parenchymatous organs, either free or encysted 

 in a connective tissue capsule ; the adults, on the contrary, live 

 principally in the alimentary canal. 



The embryo is almost invariably characterised by the special form 

 of the oral and caudal extremities, but sometimes also by the posses- 

 sion of a boring tooth, or of a circle of spines (Gordius). Sooner 

 or later the skin is shed, and the animal enters its second stage, 

 which may often still be considered as a larval stage; repeated 

 ecdyses precede the sexually adult stage. 



The post-embryonic development of the Nematodes presents 

 numerous modifications. In the simplest cases the embryo, while 



still enveloped in the egg mem- 

 branes, is transported passively 

 in the food (Oxyuris vermicularis 

 and Trichocephalus}. In many 

 Ascaridce to judge by the species 

 parasitic in the Cat the em- 

 bryos, which are provided with 

 a boring tooth, first make their 

 way into an intermediate host, 

 by which they are transported 



Fi. 2Sl.-8clerortomum tetracanthum, en- i nto tne intestine of the SCCOnd 

 eysted (after R. Leuckart). 



host with the food or water. 



More frequently the young forms encyst within the intermediate 

 host, and, enclosed in the cyst, are transferred into the stomach and 

 intestine of the permanent host (fig. 281). For example, the embryos 

 of Spiroptera obtusa of the Mouse, while still in the egg membranes, 

 are taken with the food by the Meal-worm, in the body cavity of 

 which they encyst. In the viviparous Trichina spiralis there is a 

 modification of this mode of development inasmuch as the migration 

 of the embryos and their development to the encysted form found 

 in the muscles (muscle-trichina) take place in the same animal 

 which contains the sexually mature intestinal Trichinas. 



The development of the Nematode larvse often makes a considerable 

 advance within the intermediate host into which they have migrated. 

 Thus, for instance, in Cucullanus elegans, the embryos migrate into 

 the Cyclops, and in the body cavity of these small Crustacea undergo 

 two ecdyses and essential alterations of form, obtaining at this early 



