ON THE ANGUILLULID.E. 79 



file within tlie genital tubes, though often occupying the whole width of the body. This 

 is a condition of things quite in harmony with the several requirements of animals de- 

 pendent upon such totally different conditions. The free Nematodes produce their ova 

 or young at once in that environment which they are destined to inhabit, whereas the 

 parasitic progeny are subjected to a multiplicity of chances and contingencies before 

 they meet with the necessary conditions siiitable for their development : there must be 

 many blanks in order to ensure a few prizes. It is but another instance of the harmony 

 subsisting between the observed biological history of an organized being and the physical 

 conditions to which it is suljjected and surrounded ; and the difference in this respect 

 betAveen the two divisions of the order Nematoidea may not inaptly be compared to that 

 existing between the predaceous cartilaginous fishes, on the one hand, and the ordinary 

 osseous species on the other. We may note the same limited number of progeny in those 

 forms whose young are most likely to survive, owing to their being produced viviparously 

 or else with the egg enclosed in a coriaceous envelope, which, for additional security, 

 becomes fijced by means of its tendi-ils to some rock or larger seaweed. AVhilst the ova 

 or young of such species may be numbered by units, for those of the majority of osseous 

 fishes we may substitute, instead of units, millions or even billions. 



Then many of the free Nematoids, more especially of the marine species, are provided 

 with such rudimentary sense-organs as would be useless to a parasite. These exist in 

 the form of distinct, reddish, conical and circumscribed masses of pigment, with the 

 addition occasionally of transparent lens-like bodies, situated on the anterior part of the 

 oesophagus, which doubtless subserve the purpose of rudimentary visual organs. And, 

 lastly, almost aU the free Nematodes are furnished with a caudal sucker, most highly 

 developed in the marine species, to whom its utility is obvious, by enabling their smooth 

 and polished bodies to adhere to the particular weeds Avhich they infest, whilst these 

 are swayed to and fro by the currents of the flowing and receding tide. 



These various considerations lead me to believe that the free Nematoids constitute a 

 group absolutely distinct from the parasitic forms ; and I have dwelt upon this point, not 

 only because it has not been enforced by previous writers, but also vnth the view of 

 showing the untenability of the opposite hypothesis, advanced, perhaps somewhat hastily, 

 by a most accurate observer, and one whose opinions generally are so worthy of credit. 

 On this account, too, it does not seem to me desirable to associate with these animals, as 

 Dujardin has done in his fifth section, "Enopliens," the two parasitic genera, JPassalurns 

 and Atnwtis — and this not simply on the arbitrary ground of their being parasitic, but 

 because they neither of them comply vrith those structural conditions which were stated 

 to obtain almost universally in the group in question. They appear to have been so 

 placed by Dujardin, from the simple fact of their possessing a mouth armed with three 

 teeth or jaws, which he took to be the typical character of this group, as shown by the 

 name he applied to them. But a reference to the figures and descriptions of the species 

 discovered by Dr. Eberth and myself will show that this is a structure quite exceptional 

 — only met with in one or two genera, and therefore untenable as a family distinction. 

 Diesing, also, in his recent communication on the classification of the Nematoids, has 

 associated with these animals certain parasitic genera ; and in this paper, as well as in his 



