“(c).—The organised nurses (or rediæ, as they are termed) have a mouth and a strongly marked muscular œsophagus, which is continued into a short or prolonged, single, blind intestine, or the latter may be double. The expulsion of animals developed within them I have only seen to take place through an opening at the hinder extremity. Old rediæ lose their structure. I did not observe any vascular system. Tailed trematode larvæ (Cercariæ), as well as rediæ themselves, are developed within the rediæ, this variation of nurse-contents probably depending on the season.
“(d).—No independent new germ-sacs are developed within the simple unorganised germ-sacs (sporocysts), and only such trematode larvæ as are capable of arriving at sexual maturity are furnished with special appendages.
“(e).—When the immature contents of both nurse forms (i.e. of sporocysts and rediæ) are accidentally set free, and are situated within the organs of nutrition of the living host, then they appear prepared to develop themselves anew into nurse forms; and, moreover, cercariæ whose development has not yet attained a definite stage—and even their tails also—appear to enjoy a similar capacity. Some nurses are likewise capable of multiplication by division and budding.
“(f).—Some germ-sacs have the property of developing within themselves cercaria-like larvæ—which are different from the true cercariæ—from whose body the development of a distoma may take place, while their single or double tail-like appendages in all cases develop anew into germ-sacs. To this class belong Bucephalus and Distoma duplicatum.
“(g).—All the cercaria at present known are destitute of eyes, but other forms of trematode larvæ are furnished with visual organs. Accordingly, I never found eyes in young distomata whilst they were in their last dwelling-place, but eyes are certainly present in the young forms of Polystoma and Amphistoma. The supposition that a spontaneous wandering is associated with eyes is not yet confirmed in my experience.
“(h).—As a means of distinguishing the different forms of Cercariæ, amongst other indications, their places of dwelling may be useful, because each mollusk only harbors a limited number of species. Notwithstanding, Professor Filippi is in error if he believes that every species of mollusk carries only a single armed form of cercaria. A migration of the cercaria is indispensable to its perfection.
“(i).—Many larval trematodes form cysts round themselves, probably by means of a special organ of secretion, and also by the epidermis. Their future destiny necessitates this. The sporocysts apparently fulfil towards the larvæ, which are developed within them, a similar purpose, namely, a protection against the stomachal digestion of the new host. In the pupa condition the development of the larva, which has now thrown off the tail, makes greater or less progress, according as to whether it is surrounded by nourishment or not. In particular, while in this stage, the different kinds of hooks for migratory purposes make their appearance, always, without doubt, after the shedding of the skin. Other trematodes pass through this tail-less sexually-immature stage without any cyst. I have not yet seen any larval trematode forms which had been produced in sporocysts or rediæ without appendages; they appear to occur, nevertheless.
“(k).—As the larvæ exist only in a few hosts—and most of them dwell only in one species of animal—so, also, the continued progress towards sexual maturity only succeeds in the case of certain well-defined larval organisms, but the digestion of the cysts and liberation of the larvæ may be accomplished in various animals.
“(l).—The armed Cercariæ appear to be larvæ of the spine-covered distomes of amphibia; for, as examples, the Cercaria ornata becomes transformed into Distoma clavigerum, and C. armata into Distoma endolobum; the Dist. duplicatum and Cerc. diplocotylea are, apparently, the juvenile forms of Dist. cygnoides and Amphistoma subclavatum. The Dist. echiniferum of Paludina could neither be advanced in development in the frog or duck, nor could all the other larvæ which I subjected to experiment be developed either in the green or brown frogs.
“(m).—When young trematodes arrive at the right place for their maturation, then the male generative structures develop before the female organs, and in the subsequent excess of egg production the form and structure of the animal becomes obliterated.