On the problem of the transmission of Taenia elliptica*
So as to explain my views on this matter, I would like to add the following remarks.
In vol. 2 of this publication (1887), I presented a communication affirming that transmission of Taenia elliptica is not necessarily dependent on Trichodectes, as I believe has been shown beyond doubt by Grassi's experiments (v.2, n.11, p.311). 1 I intended to offer only a small factual contribution to known observations, and in so doing express my agreement with the interpretation of the facts derived from Grassi's investigations. I have never had any intention of denying the mode of transmission indicated by Leuckart; it seems quite clear to me that Grassi's work on Taenia nana will of course prompt a complete revision of our present views on the intermediate hosts of species of cysticercoid-forming Taenia. What occurs in one species will probably be found applicable in other species as well.
Furthermore, earlier I had already interpreted Grassi's descriptions precisely as Leuckart now does in his addendum to my communication, i.e., that the cysticercoid phase is by no means suppressed but occurs inside the definitive host; in short, there is merely a chronological shift of the metamorphose to the definitive host. The possibility that in some species there may be two sites for the cysticercoid stage seems to have been demonstrated for Taenia murina, provided, that is, that there is a relationship between the mealworm and this species of tapeworm. Such an assumption also seems more likely than a simple division into species with a change of host and species without a change of host, since in the latter case the intermediate stages most likely do not occur. I purposely did not use the expression ‘direct development’, but Grassi himself seems to understand it only as the lack of an intermediate host. When comparing Taenia nana to Oxyuris vermicularis, he was probably only thinking about the fact that in both cases the eggs first leave the definitive host, and then, following a new ingestion, undergo the rest of their post-ovular development within this host. He must have realized that in two such different classes, the different stages would not coincide entirely. At least that is the way I interpreted the facts upon reading Grassi's paper. The parallel development of different generations without first leaving the intestine (in the egg stage) never occurred to me. At the most, this could only be assumed if the proglottides reached the stomach first.
I quite agree with Leuckart when he says that the elucidation of these problems can only be reached by experimentation. However, it does seem justified to take the more clinical observations into consideration, even if they do seem to run counter to prevalent views. They may not bring proof when taken singularly, but they may make up for this by their conjunction. This was my sole intent when I presented my communications.