Trematodes Sentence Examples
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.
We have, in fact, a form of larval multiplication that recalls the development of digenetic Trematodes.
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.Advertisement
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.Advertisement
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.Advertisement
These Trematodes live chiefly in the intestine of aquatic birds or reptiles.
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.
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.Advertisement
A vast number of trematodes, or poultry tapeworms, have been identified, with a wide range of hosts.
Most other trematodes are hermaphroditic, and are found in the intestinal tract, or organs such as the liver.
Trematoda (see Trematodes).
The vagina of Cestodes is undoubtedly comparable with the so-called " uterus " of Trematodes, but the nature of the Cestode uterus is not so clear.
It has been compared with the canal of Laurer of Trematodes (the vitello-intestinal duct of the ectoparasitic flukes), but if we take the more primitive Cestodes, and especially Aniphilina, into consideration we find that they possess, in addition to the uterus, an anterior vagina (usually present in Cestodes) and a posterior one.Advertisement
The single anterior vagina is then comparable with the similarly named duct of ectoparasitic Trematodes, in which group it is either single or double.
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.
The Trematodes are somewhat modified in accordance with their ectoor endoparasitic life, but they exhibit such a close similarity of structure with the Turbellaria that their origin from Planarians can hardly be doubted, and indeed the Temnocephaloidea (see Planarians) form an almost ideal annectant group linking the ectoparasitic Trematodes and Rhabdocoel Planarians.
The so-called flatworms (Platyelmia, q.v.), including the Planarians, Flukes (see Trematodes), Cestodes (see Tapeworm) and the curious Mesozoa, are no doubt related.