NEMATODA. 279 



rolled up in several coils within the shell. The excretory pore and 

 the rudiments of generative organs, as well as a nerve ring, are 

 present in the embryo, which is also provided with mouth and 

 anus. The free development is a metamorphosis, usualjy compli- 

 cated by the circumstance that it is not undergone in the habitat 

 of the mother. The young stages or larvae, probably of most 

 Nematodes, have a different habitat to that of the sexually adult 

 animal, being contained in different organs of the same or even 

 of different hosts. The larvae live for the most part in parenchy- 

 matous 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 

 possession of a boring tooth. 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 develop- 

 ment of the Nematodes presents 

 numerous modifications. In the 

 simplest cases the embryo, while 

 still enveloped in the egg mem- 

 branes, is transported passively Fia 2 ^_ sderostomum tetracanthum> 



in the food (OxyuriS vermicularis encysted (after R. Leuckart). 



and Trichocephalus). In many 



Ascaridae the embryos, which are provided with a boring tooth, 



first make their way into an intermediate host, by which they are 



transported into the intestine of the second 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. 229). 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 stage 

 found encysted in the muscles (muscle-trichina), takes place in the 

 same animal which contains the sexually mature intestinal Trichinas. 



