[7] Refer to the collected literature under _Dibothriocephalus latus_, and the reply to Küchenmeister by Braun (“Ueber den Zwischenwirt des breit. Bandw.” Würzb.: Stuber, 1886).
Naturally it does not follow that direct development in the cestodes is altogether lacking. The researches of Grassi (1889) have furnished an example in _Hymenolepis_ (_Tænia_) _murina_, which shows that development may sometimes take place without an intermediate host, notwithstanding the retention of the cystic stage. It was found that the oncospheres of this species, introduced into rats of a certain age, after a time grow into tapeworms without leaving the intestine, but not directly, for they bore into the intestinal wall, where they pass the cystic stage, the cysts afterwards falling into the intestinal lumen, where they develop into tapeworms. The recent experiments of Nicoll (1911) show that the larval stages of _Hymenolepis murina_ also occur in the rat-flea, _Ceratophyllus fasciatus_.
Important observations were soon made on the remaining groups of helminthes. The discussion on the origin of parasites soon became confined to the helminthes. Amongst the Nematoda, it had long been known that encapsuled forms existed that had at first been regarded as independent species, but very soon they were pronounced to be immature forms, in consequence of their lack of sexual organs. Though Dujardin and also v. Siebold regarded them as “strayed” animals, v. Stein (1853) very promptly demonstrated that the progeny of the nematodes were destined to travel by discovering a perforating organ in the larval nematodes of the mealworm. This was first experimentally confirmed (1860) by R. Leuckart, R. Virchow and Zenker, all of whom succeeded not only in bringing to maturity the muscle Trichinæ (known since 1830) in the intestine of the animals experimented upon, but were likewise able to follow the migrations of the progeny. Of course, the encapsulating brood remained in the same organism, and in this respect deviated from the broods of other helminthes which escape into the outer world and find their way into other animals, but the encapsuled nematodes could no longer be regarded as the result of straying. Subsequently, R. Leuckart worked out, more or less completely, the history of the development of numerous nematodes, or pointed out the way in which further investigations should be made. It has been found that in nematodes far more frequently than in other helminthes, the typical course of development is subject partly to curtailment and partly to complications, which sometimes considerably increase the difficulties of investigation and have hitherto prevented the attainment of a definite conclusion, though the way to it is now clear.