ON THE ANGUILLULID.^. 77 



of tlie genital organs from that above mentioned ; those of his second, including Filaria, 

 Spiroptera, &c., are distinguished " par la presence de deux organes copulatoires, ou 

 penis, inegaux," whilst of his third section, containing the genera Strongylus, Leptodera, 

 Dlcelis, &c., the members of which do possess two equal spicules, with or without acces- 

 sory pieces, the genus Leptodera is the only one affording also the character of a u.terus 

 divided into two equal and opposite branches, with the vulva occupying a median position. 

 This genus contains one species ', the L. flexiUs, found in the vas deferens of lAmax 

 cinereus. In his fourth section, comprising the Ascarides, the members of the second 

 su.b genus, Ascaridia, including several species found in the intestines of birds, comply 

 with the requirements so far as the male intromittent organs and double iiterus are con- 

 cerned, but differ by the presence of the three prominent cephalic lobes and the filiform 

 ovaries characteristic of the genus Ascaris. In his fifth grovip, Dujardin places these 

 free Nematodes together with certain other genera. The sixth section, containing Scle- 

 rostoma, Syngamus, &c., contains only one genus, Angiostomum, having the before- 

 mentioned arrangement of the male and female sexual organs ; but the figures given 

 by Dujardin of the only two species of this genus — one found in the lungs of Anguis 

 fragilis -, and the other in the intestine of a pulmonate Gasteropod — seem to indicate a 

 totally different formation of the ovarian tubes. The individuals of the seventh and last 

 section, including the genera Dacnitis, Opliiostoma, &c., are out of the question, from 

 their not possessing a terminal mouth ; and those of the appendix do not comply with 

 other conditions^. 



This evidence seems a sufficient warrant for the belief in the non-parasitic nature of 

 the animals in question, since it could scarcely happen, if these forms were ever para- 

 sitic, that they should not some of them — or, at all events, species of the same genera 

 — have been met with in this condition, so as to enable us to include in the same genus 

 parasitic and non-parasitic types. To me, indeed, it seems clear that these free Nema- 

 toids themselves, which can be detected in all stages of growth in external media, are 

 not likely, as a rule, to be capable of existing also as parasites. Then comes the question, 

 are they as a group distinguished by any particu.lar characters from the parasitic forms ? 

 To which I think we are fairly entitled to return an answer in the affirmative, after the 

 statements that have just been made, and from a consideration of other facts to be 

 presently mentioned'*. So far, too, this is in accordance with the views held by the 



' Another and much larger species has since been found by Dr. Baird iii the abdominal cavity of Siredon mexicanus 

 (Proc. Zool. Soc. 1858, p. 225, (Annulosa) pi. 52. f. 6, 7. 



- This species, Angiostomum entomelas, I have lately discovered, and have satisfied myself, not only that it does 

 not belong to any of the genera of free Nematodes at present known, but that it is distinguished from the members 

 of this group generally by the form of its ovaries and the extreme thinness of its integument. 



'■' The genus Odontohius being the only one concerning which there is any doubt, the nature of which will be 

 explained in the systematic portion of this memoir. 



* It is true that a few of these free Nematodes have been found within the intestines of other animals by Dujardin ; 

 but in all the cases related by him, their presence within the intestinal canal may be looked upon as accidental rather 

 than necessary, they having been swallowed, as he suggests, by these animals either with or as food. Ihus.Borylaimus 

 stagnalis was found by him in the intestine of the Carp. He has found species of the genus Rhabditis within the 

 intestine of small slugs and of the common Frog (which, in its turn, swallows the slug), as well as in the stomach of 

 several fishes, and the general cavity of the body of the Earth-worm. It would be desirable to have additional obser- 



