This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
Fig. 72. 1. Cercaria echinata? of Siebold. 2. Distoma-pupa, or Cercaria in the pupa state after it has cast off its tail and enclosed itself in a mucoid case. 3. The animal proceeding from the pupa a true Distoma, which has penetrated for a short distance into the body of the snail. 4. A "nurse" containing fully-developed Cercarise: v, the stomach. 5. A "parent-nurse " filled with partially-developed " nurses:" v, the stomach. (After Steenstrup).
(381). In various Cercariae, a copious mucous secretion is observable on the surface of the body even before the loss of the tail, and this secretion apparently increases during the efforts of the animal to cast off this appendage. As soon as the tail has been got rid of, the Cercaria begins, by extending and contracting its body, to turn itself round and round in the same spot. By this sort of movement it makes for itself a circular cavity within the mucus, which gradually hardens, and forms a tough, nearly transparent case around it. This is the noted pupa-state of the Cercariae, observed first by Nitzsch*, and afterwards by Siebold. The tailless Cercariae remain concealed under their transparent case, which is arched over them like a small, closely-shut watch-glass (fig. 72,2.) In this condition they remain some months in a quiescent and inactive state, when they present themselves with all the characters of real Flukes (fig. 72, 3), and may be found under this form lodged in the liver or appropriate viscera in the interior of the snail.
(382). Having thus seen that the Cercaria becomes an actual Fluke, it next remains to inquire what is the origin of the Cercaria. The Fluke deposits ova, from which, either within the body of the parent, or external to it, oval-shaped young proceed, which move about briskly in the fluid contained in the interior of the snail or in the surrounding water, and bear no resemblance to their parent. In what way this progeny is transformed into a Fluke, or rather, as we now know, into a Cercaria, is as yet an unexplained mystery; but that this change can and does take place only by the intervention of several generations may be assumed as beyond doubt.
(383). The free-swimming Cercariae, afterwards converted into pupae, as above described, have been proved, by the observations of Bojanus, to be produced from little worms of a bright yellow colour (fig. 72, 4) ("konigsgelben Wurmern"), described by him, and which occur in great numbers in the interior of snails, especially of Limnmus stagnalis and Paludina vivipara. It is consequently in these yellow worms, which are about 2 lines long, that the Cercariae, which are the larvae of the actual Flukes, are developed; and since we now know that the Flukes are perfect animals, which themselves undergo no transformation and are propagated by ova, we are reduced to the conclusion that the progeny is indebted for its origin and development to creatures which, in external form, and partly in internal organization, differ from the animals into which that progeny is afterwards developed; in other words, it may be said that we here meet with a generation of nurses, and that the yellow cylindrical worms of Bojanus, which inhabit the snail, are the nurses of the Cercariae and Distomata1.
* Beitrag zur Infusorienkunde, oder Naturbeschreibung der Zercarien und Bacil-larien. Halle, 1817.
1 That the Cercaria are actually developed in the above-mentioned yellow worms, any one may be easily convinced who will take a dozen large specimens of Limnceus grown "parent-nurses" is longer and wider than in any even of the youngest "nurses." (Compare fig. 72,4, with fig. 72, 5).
(384). The "nurses" usually present the appearance of the figure given above (fig. 72, 4.) The body is cylindrical, and is furnished in most instances with a spherical contracted head, which includes an oral cavity with very muscular walls and a small circular mouth. At some distance posterior to the middle of the body are situated the two characteristic oblique processes, which, as well as the part of the trunk posterior to them, are simply local dilatations of the cavity of the body. Of internal organs, there is only to be seen an undivided sacculated stomach (v), very small in proportion to the size of the animal.
The whole remainder of the very large body is filled with the brood of Cerearice. In the instance figured above (fig. 72, 4), all the embryos have simultaneously reached their full development, which is but seldom the case, since, in the same individual, Cerearice are found in all stages of development.
(385). Some doubt exists as to the mode in which the Cerearice quit their "nurses" since it has been observed, under the microscope, that there are two places where they come away, viz. from each side of the body, at a depression under the collar, and from the abdominal surface, between the two oblique processes: but they escape from the latter situation only when the animal has been slightly compressed between the glasses; and from the former, on the contrary, when no pressure at all has been employed.
(386). It next remains to trace the origin of the "nurses" themselves. Siebold (who did not regard these as independent animals, but only as living organs of generation, "germ-sacs") expresses his surprise at seeing them developed from germs which are always contained in other creatures having the same outward appearance as themselves; and Steenstrup saw, with like astonishment, that it constantly occurred, in some of the snails taken from the same places as the others, that they harboured only Entozoa which had the outward form of the "nurses" but which, instead of Cercariae, contained a progeny consisting of actual "nurses" in all stages of development. This was the case only in some, and those rather young snails, whilst all the others were inhabited by "nurses" whose progeny were true Cerearice; it cannot, therefore, be doubted that it is normal for the "nurses" to originate in creatures of similar appearance to themselves, and which are thus the "nurses" of "nurses." These "parent-nurses," however (fig. 72, 5), notwithstanding their great resemblance, were not difficult to be distinguished from the common ones; the stomach, for instance, in the fullstagnalis from small stagnant pools that have been exposed to the sun; the worms will be very readily found. They are situated not so much in the viscera themselves (the liver and reproductive organs) as in the membranes covering them, and their long bodies will be found half-floating as it were in the fluid which occupies the space between the organs, and which appears to be pure water entering through the water-canals.