Orders: Nemertini, Turbellarii, Trematodes and Hirudinei.
The second point of difference between tapeworms and Trematodes lies in the absence of a definitely demonstrable " brain."
The concentration of nervous matter and ganglionic substance at the oral end of Trematodes is equivalent to the " brain " of the Planarians, but the similar thickening in the scolex of Cestodes is by no means so certainly to be called by that name.
Those Cestodes which possess no very distinct organ of attachment (such, for example, as Gyrocotyle) have no distinct ganglionic thickening more pronounced at one end of the body than at the other; and as these are forms which have retained more primitive features than the rest, and show closer affinity to the Trematodes, it seems highly probable that the complicated nervous thickening found in the scolex, and often compared with the " brain " of other Platyelmia, is a structure sui generis developed within the limits of the sub-class.
- Diagrammatic projections to exhibit the relations of the female genital ducts in Trematodes with those in Cestodes.
A, in endoparasitic Trematodes (Malacotylea).
B, in ectoparasitic Trematodes (Heterocotylea).
TREMATODES, or flukes (as they are called from their fish-like shape), one of the three classes that compose the phylum Platyelmia.
The chief peculiarities that distinguish Trematodes from their free-living allies, the Turbellaria, are the development of adhering organs for attachment to the tissues of the host; the replacement of the primitively ciliated epidermis by a thick cuticular layer and deeply sunk cells to ensure protection against the solvent action of the host; and (in one large order) a prolonged and peculiar life-history.
The only organs that exhibit any sign of degeneration are those of sense, but in the ectoparasitic Trematodes simple eye-like structures are present and perhaps serve as organs of temperature.
Trematodes never exhibit segmentation, though a superficial annulation may occur, e.g.
In the ectoparasitic Trematodes this post-oral sucker is a complex disk placed near the hinder end and provided With suckerlets, hooks and a musculature arising from a special skeleton.
In most endoparasitic Trematodes the accessory gonopore is a median and dorsal structure.
It would seem that the Trematodes present various degrees of such adaptation, for whilst some - e.g.
The influence of Trematodes on their hosts is a varied one.
The most important of the Trematodes in its effect on man is Schistostomum (Bilharzia).
The noxious influence of Trematodes is, moreover, not confined to their mature phase of life.
The anatomical structure of the Trematodes is fairly uniform (Braun).
In the endoparasitic trematodes the uterus is the only passage by which fertilization can be effected, and in cases of cross and selfimpregnation this duct is physiologically a vagina.
The Trematodes are divided into three orders, primarily distinguished by the character of their suckers, viz.: Heterocotylea, Aspidocotylea and Malacocotylea.
- Ectoparasitic Trematodes, in which a large posterior adhesive apparatus is present and is usually accompanied by a pair of suckers placed anteriorly in relation to the mouth.
These Trematodes occur in the alimentary canal and adjacent organs of Mollusca, the gall-bladder of Chimaera, and the intestine of Chelonia and of certain fish.
Endoparasitic Trematodes with a variable adhesive apparatus.
The liver-fluke (Distomum hepaticum) unlike most Trematodes flourishes in a wide range of hosts and infects man, horse, deer, oxen, sheep, pig, rabbit and kangaroo.
These Trematodes live chiefly in the intestine of aquatic birds or reptiles.
Braun, "Trematodes," Klassen u.
From these are given off at irregular intervals short lateral branches, each of which terminates in a flame-cell (f) precisely similar in structure to the flame-cells found in Planarians, Trematodes and Cestodes; here as there the question whether they are open to the body cavity or not must probably be answered in the negative.
PLATYELMIA, a phylum of the animal kingdom which comprises three classes, the Planarians, Trematodes and Cestodes.
The Planarians are free-living animals, the Trematodes are parasitic upon and within animals, and the Cestodes are wholly endoparasitic.
Within this muscular tube lies a parenchymatous tissue which may be uniform (Cestodes) or differentiated into a central or digestive, and a peripheral portion (some Turbellaria), or finally the central portion becomes tubular and forms the digestive sac (Trematodes), while the peripheral portion is separated from it by a space lined in some forms by a flattened epithelium (most Planarians).
Their structure is similar to that of Trematodes, from which in the opinion of most zoologists they have been derived.