OF THE NEMATOIDS, PARASITIC AND FREE. 607 



investigation demanding the utmost care and patience; and although it constitutes a 

 subject replete with interest, I have not had sufficient time at my disposal to undertake it. 

 All are agreed, however, as to the direct method of development, by which the entire 

 fecundated yelk-mass, after undergoing the well-known process of segmentation, is con- 

 verted into the form of the future animal, either without, or whilst still enclosed within 

 the body of its parent. Further than this, however, almost nothing is known concerning 

 the after stages of development in which, by the development and differentiation of its 

 internal organs, it attains to the typical form of its species ; and this may be accounted 

 for in a measure by the fact that, with one recent exception only, the complete and 

 entire life-history of no other parasitic Nematoid has been fully revealed *. A gap remains 

 in the history of most between the period of their emergence from the egg, and the time 

 when they are usually met with as sexually mature individuals within the bodies of their 

 various hosts. Whether during this intervening period they are to be met with in other 

 primary intermediate hosts, in a non-sexual condition, such as we are familiar with in 

 Trichina spiralis and the species included under the name of Filariapiscittm, or whether 

 dui-ing the course of their existence they are, as a rule, parasitic only within a single 

 animal, is a question which at present we are unable to answer ; and unfortunately the 

 latest and most distinguished writer upon helminthology in this country has made such 

 opposing and contradictory statementsf as to leave us entirely in the dark as to the real 

 nature of his news on this question J. It is, however, amatter of perfect certainty as 



* Since this was written Professor Leuckabt has published an admirable paper " On the Developmental 

 History of the Nematoid Worms" (Archiv fiir "Wisscnsch. Heilkunde, Band II. pp. 195-235, and translated in 

 the Annals and Magazine of Natural History, May and June, 1866), in which he has contributed very largely 

 to our stock of knowledge on this subject. He has ascertained the complete life-history of many species of 

 parasitic Nematoids, and gives numerous details concerning developmental modifications. 



f Compare opinion expressed at p. 308 with that at p. 313 of Dr. Cobbold's ' Entozoa,' 1864. 



J Professor Leuckart (loc. cU.) has now definitely ascertained that both these methods of development exist 

 amongst the Nematoids. A synopsis of the different kinds of life-histories met with amongst these animals is 

 given by him in the BuU. de I'Acad. Eoy. des Sc. Belg., No. 3, 1866, p. 208. The most remarkable history 

 jet revealed is that of Ascaris nigrovmosa. It has been shown by Leuckart and Mecznikow that the young of 

 this animal, after passing from the rectum of the frog into damp earth or mud, grow rapidly, and actually developo 

 in the course of a few days, whilst still in this external medium, into sexudtty mature animak. Young, differing 

 somewhat in external characters from their parents, are soon produced, and these attain merely a certain stage 

 of development whilst in the moist earth, arriving at sexual maturity only after they have become parasites, 

 and are ensconced in the lung of the frog. Here, as Professor Lexjckaet remarks, we meet with " no simple 

 alternation of the conditions of life, but with an alternate sequence of free and parasitic generations. And 

 what is most wonderful, hoth these generations are seooually developed, both are produced from ova. Here, 

 therefore, we have nothing to do with an ordinary alternation of generations, such as occurs, for example, in the 

 Distomece, but with a process hitherto almost unheard of in the animal kingdom, and which calls for our con- 

 sideration the more because we are accustomed to regard the sexual development of an animal not merely as 

 the sign of its perfect maturity, but also as the criterion of specific individuality." The life-history of this animal 

 will be more marvellous still if Professor Leuckabt's supposition be correct (and it seems the most probable one) 

 as to the young of the parasitic form being produced by a process of agamogenesis. No parasitic males of this 

 species have ever yet been met with. 



