The Tenia soliam is subject to many varieties of form or malformations; the head has been ob served to present six oscula instead of four. In the Imperial Museum at Vienna, so celebrated for its entomological collection, there is a por tion of a Tania sulium, of which one of the margins is single and the other double, as it were two ti•niue joined by one margin. In the Museum of the College of Surgeons is preserved a fragment of the Tmnia solium of unusual size; it swells out suddenly to the breadth of three fourths of an inch with a proportionate degree of thickness, and then diminishes to the usual breadth.'" The species of Tania infesting the intestines of other animals are extremely numerous, ne vertheless they are rare in Fishes, in which they seem to be replaced by the Bothriucephati and Liguter. The determination of the species in this, as in every other natural and circumscribed genus, is extremely difficult and often uncer tain: their study is facilitated by distributing them into the three following sections, of which the first includes those species which are de prived of a proboscis, rtrIliX inermes ; the second those which have a proboscis, but un armed, Thuile rostellata.; the third the Tape-worms with an uncinated proboscis, !Lanier ar ma ter.
The Trematode Order, which is the most extensive division of the Parenchymatous class of En tozoa, and embraces the greatest number of generic forms, in cludes only two species infesting the human body, one of which, the liver-fluke ( Distatna heputi cum), is extremely rare, and the other ( Polystama Pingnicula) somewhat problematical.
The Distaina hepatieum (fig. 64) is found in the gall-bladder and duets of the liver of a variety of quadrupeds, and very com monly in the Sheep. When it occurs in the I Inman species, it is generally developed in the same locality. The form of this species of En tozoa is ovate, elongate, flattened ; the anterior pore or true mouth (a) is round and small, the posterior cavity (b), which is imperforate and subservient only to adhesion and locomotion, is large, transversely oval, and situated on the ventral surface of the body in the anterior moiety. Between these cavities there is a third orifice (c) exclusively destined, like the orifice on each joint of the Tmnia, to the generative system ; and from which a small cylindrical process, or term:isms, is generally protruded in the full-sized specimens.
The form of the •body is so different in the young Distornata, that ltudolphi was induced to believe the specimens from the human gall bladder which were in this state, to belong to a distinct species, which he termed laneeo latum ; this modification, which is wholly de pendent upon age, is shown in the subjoined figure ; and we shall hereafter have to notice the more extraordinary changes, amounting to a metamorphosis, which the Distumata infesting the intestines of Fish undergo.
The Palystorna Pinguieola was discovered by Treutler, in the cavity of an indurated adi pose tubercle, in the left ovarium of a female, a:tat. 20 ; it is represented in situ, at A, fig. 62.
Its natural size and shape is shewn at B, the body is depressed,subconvexabove, concave below, subtruncate anteriorly, a little contracted behind the head, pointed at the posterior extremity. On the under side of the head C, there are six orbicular pores disposed in a semi lunar form : a larger sucto rictus cavity occurs on the ventral aspect at the begin ning of the tail (b B), and a small orifice is situated at the apical extremity.
A second species of Po lystoma ( Palystama Irma rum), stated by Treutler to have been situated in the anterior tibial vein of a Man, which was accidentally ruptured while bathing, is generally supposed to have belonged to a species of Planaria, and to have been acci dentally introduced into the strange locality above-mentioned.
The worms of the Trematode order are those which are most frequent in the interior of the eyes of different animals, perhaps the most singular situation in which Entozoa have as yet been found, and respecting which much in teresting information bas recently been given by Dr. Nordmann, in the first part of his beautiful work entitled " Mikrographische Beitriige zur Naturgeschicte der Wirbellosen Thiere." Of the species described and figured in that work, we have selected for illustration the Diplastumum valverm.
Fig. 66 exhibits a magnified view of the vitreous humour of a Perch ( Perca 1.inn.) containing numerous specimens of this parasite, which sometimes exists in such pro digious numbers, that the cavity of the eyeball is almost exclusively filled by them. They not only infest the vitreous but also the aqueous Humours, and have been found in the choroid gland.
All the species of Diplostomum are very small, seldom exceeding a sixth part of a line in length. They resemble the genus Distoma, and present some affinity to the Cercarie, which infest the fresh-water Snails ; but they have characters peculiar to themselves which entitle them to rank as a distinct genus ; of these the principal external one is the addi tional sucker developed on the ventral aspect of the body, as compared with Distoma, whence Nordmann calls the genus Diplosto main, though Diplo-cotylus would be the more appropriate designation, since, as before ob served, the ventral depressions are simply organs of adhesion, and have no communication with the alimentary canal. Besides the suckers the Diplostomum has an anterior mouth (a,fig. 81), as in the Distoma. The first or anterior sucker (b,fig. 81) is twice the size of the mouth; and the second (c,fig.81) is again double the size of the former. As the figure shows the vessels from the dorsal aspect, these suckers can only be seen in outline. The animal has great power over them and can contract the parenchyma of the body surrounding them, so as to make them project like rudimental extremities from the ventral surface.