Some species of the Trematode Entozoa are infested by parasitic Polygastrtca which belong to the Monads : Nordmann observed some brown corpuscles by the sides of the alimen tary canal of a Diplostomum, which contained minute particles in continual and lively motion. On crushing the corpuscles between plates of glass an immense concourse of the moving atoms escaped : they were smaller than tbe lifonas atomos of Muller, of an oval form, and of a clear yellow colour; their movements were very singular : they whirled rapidly round on their axis, then darted forward in a straight line, whirled round again, and again darted forward. When we consider that the Diplos tomum itself does not exceed a quarter of a line in length, and that the aqueous humour of a single eye serves as the sphere of existence to hundreds of individuals, what views does the fact of the parasites of so minute an Ento zoon open of the boundless and inexhaustible field of the animal creation I Acanthocephala.—The worms of this order, although in external form, in the development of the tegumentary and muscular system, and above all in their dicecious generation, they ap proach very closely the Nematoid \Vorms, yct preserve the distinguishing character of the Sterelminthoid class in the structure of the digestive organs. In the Echinorhynchus gigas the mouth is an extremely minute pore, situated on a projectile armed proboscis, the structure of which we have already described. From its posterior part are continued two long cylindrical canals (e, e, figs. 83,84) which ad here closely to the muscular fibres by their outer side, and project on the opposite side into the triangular cavity (h, fig. 84) left between the ovaries in the female and testes in the male. They arc extremely minute at their commencement, but increase so as to be readily visible in the middle of their course. They are trans parent and irregularly dila ted or sacculated at inter vals. Posteriorly they ter minate in a cul-de-sac, and have no anal outlet. They contain a transparent in odorous albuminous liquid, give off no visible lateral branches, and do not com municate together in any part of their course. Be sides these canals we find in the cavity of the body of an Echinorhynchus two long wavy tubes called lemnisci, (d, d, fig. 83). They are attached to the lateral parts of the neck by an extremely attenuated an terior extremity, float freely in the remainder of their extent, and terminate in an enlarged obtuse and imper forate extremity. They are of a whitish colour, tran sparent in the living worm, hut become opake after death; they present consi derable variety of form, and would seem to be highly irritable parts, since they are not unfrequently found fold ed into a packet, or twisted both together, and turned to one side of the body. When examined with a high microscopic power, a tran sparent vessel is perceived running through the centre and ramifying as it descends in the substance of the lem niscus, which is soft, fragile, and granular. Cloquet com pares these organs to the nutrient processes which project into the abdominal cavity of the Ascaris, and they are also regarded by Goeze, Zeder,and Rudolphi as belonging to the organs of nutrition.
In the Crelelmintha or Cavitary Entozoa, the mentary canal is single and of large size, and extends nearly in a straight line from the mouth to the anus, which are at opposite extremities of the body. With regard to the existence of an anal outlet, the parasitic Entozoon, ( Syngamus Irachealis, Siebold,) which infests the windpipe of our common Gallinaecoos Birds, presents an exception. it was supposed by Montague to be
a single ind ivid ual with too peffimculate mouths: and by Rudolphi was placed in the same group as Distoma furcatum, which is a true double necked Trematode worm. But the digestive system has the essential character of the ccelel minthic structure, the intestine floating freely in an abdominal cavity. The orifice at the extremity of the smaller or male branch leads to a muscular cesopliag-us, which is continuous with a somewhat broader reddish-brown intes tine, continued in a tortuous manner down the neck, and terminating in a cul-de-sac prior to the confluence of the extremity of this branch with the body of the female. The mouth of the larger branch, which is the true continua tion of the larger and single body, leads first to a horny basin-like cavity, which communi cates by an opposite pore, surrounded by six horny hooks or teeth, with the oesophagus, from which a similar reddish-brown intestine is continued, but in a more tortuous manner than in the male, through the whole body, ter minating in a cul-de-sac at the caudal extre mity. In both intestinal canals are molecules of apparently the colouring matter of blood. Their inner surface is reticulate.
In the freedom of these intestines from the muscular parietes of the body, and in the cy lindrical form of the latter, we have a close affinity to the Nematoid type: but the intestine is blind—without an anal outlet. It is not, however, bifurcate, as in the true Trematoda.
In the genus Linguatula or Pentastoma of Rudolphi, the intestine is a simple straight tube, and is surrounded by the convolutions of the oviduct : the two intestinula cacti with which Rudolphi describes the alimentary canal as being complicated,* appertain to the gene rative 'system, and communicate exclusively with the oviduct : the intestine terminates by a distinct anus at the posterior extremity of the body.
In the Nematoidea the intestine is also frequently concealed in a part of its extent by the coils of the genital tubes, but these are disposed in masses by the side of the alimen tary canal, and not wound around it as in the Linguatula : in most species the alimentary canal is attached to the internal parietes of the abdominal cavity by means of numerous small laminated or filamentary processes.
In the Strangylas gigas the mouth (Adis. 71) is surrounded by six papilla ; the cesopha gus (b, fig. 95) is round and slightly contorted, and suddenly dilates at the distance of about two inches from the mouth into the intestinal canal; there is no gastric portion marked off in this canal by an inferior constriction, but it is conti nued of uniform structure, slightly enlarging in diameter to the anus. The chief pecu liarity of the intestine in this species is that it is a square and not a cylindrical tube, and the mesenteric processes pass from the four longitudinal and nearly equidistant angles of the intestine to the abdominal parietcs. These processes, when viewed by a high mag nifying power, are partly composed of fibres and partly of strings of clear globules, which appear like rnoniliform vessels turning around the fibres. The whole inner surface of the abdominal cavity is beset with soft, short, obtuse, pulpy processes, which probably im bibe the nutriment exuded from the intestine into the general cavity of the body, and carry it to the four longitudinal vessels, which tra verse at equal distances the muscular parietes. The analogous processes are more highly de veloped in the Ascanis lumbricoides, in which species we shall consider the digestive and nutritive apparatus more in detail.