The Entozoa of the parenchymatous class are chiefly oviparous, those of the cavitary class for the most part ovoviviparous.
The germinal vesicle has not yet been dis covered in the vitelline substance of the ova of the Acanthocephala, Treinutoda, or Ces toidca ; but it is distinctly discernible in the ova of the Nematoidea ; I have also observed and have figured it in the highly organized ovum of the Linguatula tenioides.
The ova of the Denise present considerable varieties of size and form in different species ; Rudolphi has figured seven forms of these ova in the Synopsis Entozoorum, (tab. hi)* Some are much elongated and pointed at both extremities, others elliptical : the ova of the Bothriocephalas tutus are of the latter form, (CA 89); those of the Maio soliam are sphe rical, as are also the ova of Twnia In some species the development of the em bryo Tape-worm has been observed to have distinctly commenced in the undischarged ova, as in the famia palymorpha. In dissecting a Touraco infested by the Beni(' jilynrmis, we found that the segments of the Twnia in which the ova were must developed had been de tached from the rest of the body, a process remarkably analogous to that which takes place in the Lernee and Entomostraca, where the external ovaries are cast off, when charged with mature ova.
A few of the Trematode Entozoa, as the Monostoma mutabile, produce the young alive ; but these have a very different form from the parent. It would seem that they were des tined to pass a transitional state of their ex istence in a fluid medium permeated by light, since two coloured ocelli have been discovered on the head, and the surface of the body is beset with locomotive vibratile cilia.fi The ova of the greater part of the Tremolo(hi are excluded prior to the full development of the fietus ; they are generally of an oval but some times spherical form, and many of them singu larly resemble the seeds or capsules of certain mosses, in having a small circular portion of the outer covering separate from the rest, and closing the cavity of the egg like a lid.
Nordmann has studied the development of the young of the Distoma Mans, which infest the intestines of the perch. According to this
excellent observer the tcetus raises, in its en deavours to slip out of the egg, the small lid, and writhes about for some time, being still attached to one point of the egg. In about six hours it succeeds in freeing itself from the egg-coverings; and at this period it differs in every respect from the shape of the parent animal; the body, which is of a mucous con sistence and perfectly transparent, is of an oval form ; the anterior mouth forms a small square shaped projection, and the whole surface of the body is beset with many longitudinal rows of short cilia, which are in rapid and incessant motion, and create a vortex in the surrounding water, similar to that which the I'olygastric Infusoria produce. The little animal having its anterior extremity diminish ing to a point, is well formed for swimming, and by means of its vibratile cilia, quickly darts out of the field of vision when under the microscope. At the distance of one-third of the body from the anterior extremity there is a single coloured eye-speck, from which, when pressed between glass plates, there escapes a brilliant blue-coloured pigment. Thus orga nized, the young of the intestinal parasite just described move to and fro in water as if this were their natural element, and approximate in form and structure most closely to the Poly gastric Infusoria of the genus Paramecium, Ehrenb. In this state, doubtless, they are ejected by the Fish, in the intestines of which they were originally developed, into the sur rounding water,and when again received into the alimentary canal undergo their metamorphosis, lose, like the Lerma: and Cirripedes, the organ of vision which guided t4e movements of their young and free life, and grow and procreate at the expense of the nutrient secretions with which they are now abundantly provided.
In the Ca/c1 nintha the young cast their in tegument, and would seem in sonic species, as the Filaria Malincnsis, to undergo a change in the form and proportions of the extremities of the body, but they do not possess cilia or ocelli, as in the Trematoda above-mentioned.