OF THE NEMATOIDS, PAEASITIC AND FEEE. 611 



cimens of free Nematoids may frequently be seen which have attained more than half 

 the usual adult dimensions, with well-formed tissues and a fully developed alimentary 

 canal, though still in an asexual condition and presenting no trace of genital organs. 

 What is the order of development of these organs in the male, and how soon the spicules 

 appear I am unable to say, though I have been able to notice a few facts concerning the 

 genesis of this system in the female. Its first rudiments consist of a small mass of 

 indifferent tissue lying within the parietes of the body, opposite the future situation of 

 the vulva, and to a certain extent pressing upon the alimentaiy canal*; and, as I have 

 seen in species of the genus Mononchus, this gradually increases in size, more especially 

 in a longitudinal direction, its central portion growing outwards so as to come into rela- 

 tion with a gradually formed aperture through the integument — the future vulva — whilst 

 on either side it is developed into an elongated pyriform mass. So far only have I traced 

 this process of development; the remaining steps consisting, in all probability, in the 

 formation, on each side, of an internal cavity m what was at first a mass of solid homo- 

 geneous tissue ; the continued growth and separation of the parts into ovary, oviduct, 

 and uterus ; and the final difi'erentiation of tissue by which the textxu'es proper to each 

 segment are produced — these stages though easUy imagined have yet to be observed. 



With regard to the dui-ation of life in the parasitic Nematoids we have, I believe, no 

 definite knowledge ; great variation in this respect would doubtless be met with in dif- 

 ferent species. Amongst the free forms, however, we do know, as pointed out by M. 

 DAVAiXEf, that the duration of the active life of Tylenchus tritici can only be from nine 

 to ten months. In the animals of this species, too, the females die after the production 

 of a single batch of ova$ ; probably this, however, may simply be due to the necessities 

 of their mode of existence, which is so exceptional in its nature as to render any data 

 we may possess concerning these animals of little value for the determination either of 

 the period of existence, or of the number of batches of ova produced by the free Nema- 

 toids generally. 



POWEES OF EEPAIE AND TENACITY OF LIFE. 



Almost all the observations I have now to make refer to the AngidllulidcE, since what 

 we know on these subjects concerning the parasitic Nematoids may be stated in a very 

 few words. 



With regard to the powers of repair in the parasitic species I believe absolutely 

 nothing is kno-wn, but some remarkable statements have been made concerning their 

 tenacity of life, and concerning the conditions under which development of their 

 ova will proceed. Thus Nelson and Allen Thompson have best been able to study 



* This condition may be well seen in the young of Ti/Jenchus (Vibrio) tritici, -where it forms what appears a 

 clear space, with a slight displacement of the intestine opposite it, which from its shape has been termed tho 

 " lunule." The rudiments of tho genital organs are seen unusually early in this species. 



t Recherches sur rAngnillulc du ble niello, Paris, 1857, p. 38. 



t This may be considered as the most prolific of the free Nematoids ; in this respect, indeed, as well as in 

 the nature of its habitat, it approximates closely to its more strictly parasitic kindred. 



4p2 



