OKIGIN OF INTERMEDIATE HOSTS. 115 



brata became separated as a distinct phylum ; some, even in relatively 

 recent times, such as the Trichinae and others, whose life-cycle is 

 limited to mammals, most recently of all creatures. In many cases 

 the origin of new Helminths may have gone hand in hand with the 

 transformation, by means of which the hosts have gradually become 

 new species. 



That the change of a sexually mature animal into a mere pre- 

 paratory stage (a larva) the process which we have adopted to 

 elucidate the change of hosts is biologically possible, cannot be 

 doubted in view of the analogy of the so-called abbreviated develop- 

 ment, frequently mentioned above, and whose counterpart it forms. 

 If a series of different developmental phases may contract into a single 

 continuous process, then, conversely, this latter can also spread itself 

 out into a number of such phases. This is a process to which we 

 must attribute a very important role in the formation of species ; for 

 the present larval forms are to be considered, agreeably with the 

 doctrine of descent, as the original sexually mature ancestors of those 

 species which to-day represent their ultimate condition. The sum of 

 the characters by which these latter differ from the larvae represents 

 the gain which the original animal has gradually acquired under the 

 changed relations of life, changes which become, as it were, added on 

 to earlier ones, so that the development is protracted, and sexual 

 maturity, which coincides with the conclusion of development, is 

 delayed. 



The nature of those Entozoa, which are parasitic in invertebrates in 

 a mature condition, lends a yet more definite support to our supposi- 

 tion. They are, of course,, few in number, if we except the entozootic 

 Isopoda and a few others, and confine ourselves to the true Hel- 

 minths ; these belong mostly to the thread-worms. But we may 

 mention also a Trematode living in the fresh-water mussel (Aspido- 

 gaster conchicola), and a Cestode (Archigetes Sieboldi), described re- 

 cently by me, and found in the body-cavity of Scenuris. 



All these forms develop, so far as we know their life-history (p. 70), 

 without an intermediate host, and attain their sexual maturity im- 

 mediately in the first host, as would naturally be the case provided 

 our supposition were correct. 



In addition, the development and metamorphosis of these forms 

 are very simple, so that the respective animals are but little removed 

 from their hypothetical original form, and become sexually mature in 

 a condition which in many respects stands on a par with the young 

 and larval forms of their further advanced relatives. Thus the Nema- 

 todes, sexually mature, found in invertebrates (mostly omnivorous 

 insects and millipedes), follow closely the Khabditidse (Oxyuris) in 



