374 THE ANIMAL PARASITES OF MAN 



the thoracic duct, the natural connection between the vascular system 

 and the lymphatic system, to wander further independently, and 

 ultimately reach the muscular system, in which they become encysted 

 (fig. 269). Thus the progeny does not leave the body of the host 

 inhabited by the parents, as is generally the case amongst helminthics, 

 but uses it as an intermediate carrier to reach another host, which 

 is then the final host. The latter may belong to another species, 

 or may be another individual of the same species. This second 

 migration is, of course, purely passive. 



CLASSIFICATION OF THE NEMATODA. 



The Nematodes are usually divided into a number of families, some of which 

 it is at present impossible to define accurately ; moreover, the definition of many 

 genera is also in an unsatisfactory state. 



Family. Anguillulidae, Gervais and van Beneden, 1859. 



A "family" name not definable. They comprise a vast number of small forms, 

 most of which live free in fresh water, in soil, or in macerating substances ; amongst 

 them there are some which live parasitically on plants, more rarely on animals. 

 They do not exceed 8 mm. in length. The large majority are only I to 2 mm., or 

 even 0*5 mm. The uterus is straight. Eggs in the uterus at one time, one to 

 four. Genera very numerous, but many of them insufficiently defined (Anguillula, 

 Anguillulina, Rhabditis, Heterodera, etc.). 



Family. Angiostomidae, Braun, 1895. 



Small Nematodes undefined morphologically, but characterized by heterogony, 

 i.e., there is a free-living "rhabditic " generation and a parasitic " filariform " genera- 

 tion which succeed one another (e.g., Angiostomum, Strongyloides, Probstmayria). 



Family. Gnathostomidae. 



Cuticle covered totally or partly with cuticular laminae fringed posteriorly with 

 multiple points. Head subglobular, covered with simple spines. Two spicules. 

 Vulva behind middle of body, parasitic in vertebrates, especially mammals (e.g., 

 Gnathostoma, Tanqua, Rictularia). 



Family. Dracunculidae, Leiper, 1912. 



Males very small in proportion to females. Anus absent. Vulva absent (?). 

 Genera : Dracunculus, Icthyonema (in body cavity of eel and other fish). 



Family. Filariidse, Clans, 1885. 



Long thread-like Nematodes. Anus present. (Esophagus without bulb. 

 Vulva usually in anterior half of body. Two ovaries. Generally ovoviviparous. 



