ORDO V. NEMATOIDEA, Ompta,fi/Lint, gicSo;, forma.) Vermes teretes, Rund-wormer. Round WOrMS.
Char. Body elongated, rounded, elastic. Mouth variously organized according to the genera. A true intestinal canal terminating by a distinct anus. Sexes distinct.
Obs. The internal character which Itudolphi has introduced in his definition of this Order,* viz. that derived from the structure of the ali mentary canal, its free course through the body, and its termination by a distinct anus at the extremity opposite the mouth, is one of much greater value than any of the external modifica tions of the body which characterize the four preceding orders. It is, in fact, a trait of or ganization which is accompanied by corre sponding modifications of other important parts, more especially the nervous system.
The Entozoa which manifest this higher type of structure form in the system of Cuvier a group equivalent to that which is constituted by the four other orders combined. The En tozoa composing the first four orders above characterized have no distinct abdominal cavity or intestine, but the digestive function is carried on in canals without an anal outlet excavated in the parenchymatous substance of the body, and Cuvier accordingly denominates them the Vers inlestinaux pureachyntateux. The Ne niataidea, with which Cuvier rightly associates the genus Pentastoma of Itudolphi, and also (but less naturally) the Vers rigidules of La marck, or Epizoa, he denominates Vers lutes tinaux cavitaires.' With respect to the Epizoa, or the external Lernwan parasites of Fishes, although they agree with the Nematoidea and all inferior Entozoa in the absence of distinct respiratory organs, yet the ciliated natatory members which they possess in the young state, and the exter nal ovarian appendages of the adult, are cha racters which raise them above the Entozoa as a distinct and higher class of animals, having intimate relations with the soft-skinned Sipho nostornons Crustaceans.
Limiting, then, the Cavitary Entozoa to the Nematoidea of Itudolphi, and the Genera Lin guattila, Pentastoma, Porucephalas, and Syn gums, which, under the habit of Cestoid or Trematode Worms, mask a higher grade of organization, we propose to regard them as a group equivalent to the Sterelmintho, and to retain for them the name of Celelmintha.
The class of Entozoa thus constituted em braces already the types of three different orders, of which one is formed by the Nentn toidea of Rudolphi, a second has been esta blished by Diesing for the genus Pentastoma and its congeneric forms, under the name of Acanthotheco ; and the singular organization of the Soganius of Siebold, presently to be described, clearly indicates the type of a third order of Cavitary Entozoa.
As a short description has already been given of the species of Protelmintha which inhabit the human body, we shall proceed to notice those species belonging to the two di visions of Entozoa above defined, which have a similar locality, before entering upon the organization of the class generally.
The first and simplest parasite which de mands our attention is the common globular Ilydatid, which is frequently developed in the substance of the liver, kidney, or other abdo minal viscera, and occasionally exists in prodi gious numbers in dropsical cysts in the human sullect.
Considerable diversity of opinion still exists as to the nature of these ambiguous productions, to which Lacnnec first gave the name of Ace phalocysts; we shall nevertheless admit them into the category of human parasites, for reasons which are stated in the following descrip tion.
The Acephalocyst is an organized being, consisting of a globular bag, which is com posed of condensed albuminous matter, of a laminated texture, and contains a limpid co lourless fluid, with a little albuminous and a greater proportion of gelatinous substance.
The properties by which we recognize the Acephalocyst as an independent or individual organized being are, first, growth, by intrinsic power of imbibition ; and, secondly, reproduc tion of its species by gemination. The young Acephalocysts are developed between the layers of the parent cyst, and thrown off either inter nally or externally according to the species.