CLASSIFICATION.—The separation of the Nematomorpha from the Nematoda depends mainly on the character of the nervous system, the absence of the lateral lines and of the dorsal line, the character of the contents of the body-cavity, and the character of the reproductive organs. In Gordiidae the latter are always placed dorsal to the intestine, and ovaries and testes open alike at the hinder end of the body. The importance of the differences in the organs just enumerated has been considered sufficient to justify the removal of the Gordiidae from the Nematoda, and the establishment of the special sub-Order Nematomorpha for their reception; and although _Nectonema_ has a dorsal line, and is in some other respects intermediate between the two groups, there can be little doubt that it is more closely allied to _Gordius_ than to any member {170}of the Nematoda, and it must therefore be placed with it in the Nematomorpha. On the other hand, it ought to be mentioned that Camerano[210] found that the chief details of the fertilisation and development of the egg in _Gordius_ closely conform with what is known of the same processes in Nematodes, and he is of opinion that these resemblances are sufficiently important to justify the retention of the group among the Nematoda. LIFE-HISTORY.—The life-history of _Gordius_ comprises four stages—the early development of the egg, the first larval form, the second larval form, and the sexually mature form. Both larval forms are parasitic, and during their life they are actively engaged in feeding; the free form, on the other hand, takes in no nourishment, and is exclusively engaged in reproduction. [Illustration: FIG. 88.—Abdomen of _Pterostichus niger_ with the terga removed to expose the _Gordius_ larva within. Slightly magnified. (From von Linstow.)] Von Linstow[211] gives the following account of the life-history of _G. tolosanus_, a form which has been more fully worked out than any other. In the month of April numerous specimens of the beetle _Pterostichus niger_ were found floating on the surface of the ditches and small ponds in the fields surrounding Göttingen. Some were found dead or dying; others appeared quite healthy, and these were swimming actively, endeavouring to reach land. Within the abdomen of these beetles, in about 20 per cent of those collected, the second larval form of the _G. tolosanus_ was found. The longest larvae were 122 mm. in length, and very soft, partly snow-white and partly brown in colour; traces of the boring apparatus of the first larval form were still to be seen, but in other respects the larva only differed from the free form in the immaturity of its sexual organs. Besides the parasite hardly anything was to be found in the abdomen of the beetle, the larva having eaten up all trace of the fat body and the generative organs of its host. The larvae bored their way out of the body of the beetle and became adult animals.