The use of the microscope also revealed a large number of very small organisms in the water and moist soil, some of which undoubtedly resembled helminthes. Considering the wide dissemination of these minute organisms, it was natural to conjecture that after their almost unavoidable introduction into the human system they should grow into helminthes (Boerhave, Hoffmann). Linnæus went even further, for he traced the descent of the liver-fluke of sheep from a free-living planaria (Dendrocœlum lacteum), the Oxyuris vermicularis from free-living nematodes, and the Tænia lata (i.e., Dibothriocephalus latus) from a tapeworm (Schistocephalus solidus) found free in the water. Linnæus’ statements met with general approval. However, we must bear in mind that at that time the number of helminthes known was very small, and many of the forms that we have long ago learned to differentiate as specific were then regarded as belonging to one species. Linnæus’ statements were partly supported by similar discoveries by other investigators, such as Unzer, and partly also by the discovery of eggs in many helminthes. It was believed that the eggs hatched in the outside world gave rise to free-living creatures, and that these, after their introduction into the intestine, were transformed into helminthes. By means of these eggs the old investigators tried to explain the HEREDITARY TRANSMISSION of the intestinal worms, which was universally believed until the commencement of the last century. Some authors went so far as to regard the intestinal worms as congenital or inherited; they maintained the possibility of direct transmission, as in suckling, and denied that the eggs reaching the external world had anything to do with the propagation of the parasites.
The more minute comparison between the supposed free-living stages of the helminthes and their adult forms, and the impossibility of finding corresponding free forms for the ever-increasing number of parasitic species, revealed the improbability of Linnæus’ statements (O. Fr. Müller). It was the latter author also who recognized the origin of the tapeworms (Schistocephalus, Ligula) found free in the water. They originate from fishes which they quit spontaneously.
However, in spite of the fact that van Doeveren and Pallas correctly recognized the significance of the eggs in the transmission of intestinal worms, these statements remained disregarded, as did Abildgaard’s observation, experimentally confirmed, that the (immature) cestodes from the abdominal cavity of sticklebacks became mature in the intestines of aquatic birds. Moreover, at the end of the eighteenth and the commencement of the nineteenth centuries, after helminthology had been raised to a special branch of study by the successful results of the investigations of numerous authors (Goeze, Bloch, Pallas, Müller, Batsch, Rudolphi, Bremser), many of whom experienced a “divine joy” in searching the intestines of animals for helminthes, some authors reverted to generatio æquivoca, without, however, entirely denying the existence of organs of generation and eggs. The fact that a few nematodes bore living progeny—a fact of which Goeze was already aware—had no influence on the erroneous opinion, as in such cases it was considered that the young continued to develop beside the old forms. There were also many helminthes known that never developed sexual organs and never produced eggs, and which therefore were referred to generatio æquivoca. People were convinced that the intestinal mucous membrane or an intestinal villus could transform itself into a worm, either in a general morbid condition of the body, or in pathological changes of a more local character. The appearance of helminthes was even regarded as useful and as a means for the expulsion of injurious matter.
These views, firmly rooted and supported by such eminent authorities as Rudolphi and Bremser, could not easily be overthrown. First, a change took place in the knowledge of the trematodes. In 1773, O. Fr. Müller discovered Cercariæ living free in water. He regarded them as independent creatures and gave them the name that is still used at the present time. Nitzsch, who also minutely studied these organisms and who recognized the resemblance of the anterior part of their bodies to a Fasciola, did not, however, arrive at a correct conclusion. He regarded the combination rather as that of a Fasciola with a Vibrio, for which he mistook the characteristic tail of the cercaria. He also noticed the encystment (transformation into the “pupa”) on foreign bodies of many species of these animals, but was of opinion that this process signified only the termination of life.
Considerable attention was attracted to the matter when Bojanus first published a paper entitled “A Short Note on Cercaria and their Place of Origin.” He pointed out that the cercariæ creep out of the “royal yellow worms,” which occur in freshwater snails (Limnæa, Paludina), and are probably generated in these worms.
Oken, in whose journal, Isis (1818, p. 729), Bojanus published his discovery, remarks in an annotation, “One might lay a wager that these Cercariæ are the embryos of Distomes.” Soon after (1827), C. E. v. Baer was able to confirm Bojanus’ hypothesis that the cercariæ as a “heterogeneous brood” originated from spores in parasitic tubes in snails (germinating tubes). Moreover, Mehlis (Isis, 1831, p. 190) not only discovered the opercula of the ova of Distoma, but likewise saw the infusorian-like embryo emerge from the eggs of Typhlocœlum (Monostomum) flavum and Cathæmasia (Distoma) hians. A few years later (1835) v. Siebold observed the embryos (miracidia) of the Cyclocœlum (Monostomum) mutabile, and discovered in their interior a cylindrical body that behaved like an independent being (“necessary parasite”), and was so similar in appearance to the “royal yellow worms” (Bojanus) that Siebold considered the origin of the latter from the embryos of trematodes as, at all events, possible. Meanwhile, v. Nordmann of Helsingfors had in 1832 seen the miracidia of flukes provided with eyes swimming in water; v. Siebold (1835) had observed the embryos, or oncospheres, of tapeworms furnished with six hooklets in the so-called eggs of the Tænia; while Creplin (1837) had discovered the “infusorial” young of the Diphyllobothrium (Bothriocephalus) ditremum, and conjectured that similar embryos were to be found in other cestodes with operculated eggs. At all events, the fact was established that the progeny of the helminthes appeared in various forms and was partly free living. The researches of Eschricht (1841) were likewise of influence, as they elucidated the structure of the Bothriocephali, and proved that the encysted and sexless helminthes were merely immature stages.
J. I. Steenstrup (1842) was, however, the first to furnish explanations for the numerous isolated and uncomprehended discoveries. Commencing with the remarkable development of the Cœlenterata, he established the fact that the Helminthes, especially the endoparasitic trematodes, multiply by means of alternating and differently formed generations. Just as the polyp originating from the egg of a medusa represents a generation of medusæ, so does the germinal tube (“royal yellow worm”) originating from the ciliated embryo of a Distoma, etc., represent the cercaria. These were consequently regarded as the progeny of trematodes, and Steenstrup, guided by his observations, conjectured that the cercaria, whose entrance into the snails he had observed accompanied by the simultaneous loss of the propelling tail, finally penetrated into other animals, in which they became flukes.
Part of this hypothetical cycle of development was erroneous, and in other particulars positive observation was lacking, but the path pursued was in the right direction. Immediately after the appearance of Steenstrup’s celebrated work, v. Siebold expressed his opinion that the encapsuled flukes certainly had to travel, i.e., to be transmitted with their bearers into other hosts, before becoming mature. This view was experimentally confirmed by de Filippi, La Valette St. George (1855), as well as by Pagenstecher (1857), while the metamorphosis of the ciliated embryo of Distoma into a germinal tube was first seen by G. Wagener (1857) in Gorgodera (Distoma) cygnoides