[254] This grouping goes back to the year 1800, and was made by J. G. H. Zeder, a physician and helminthologist of Forchheim, who divided the helminths, which until 1851 were generally regarded as a special class of animals, into the groups of round, hook, sucker, tape and bladder worms, as which they are recognized up to the present time. In 1809, K. A. Rudolphi gave them the names Nematodes, Acanthocephali, Trematodes, Cestodes and Cystici.
[255] A sucker or acetabulum (little cup) is a round, cup-shaped muscular organ, the muscles of which are sharply defined from those of the body.
[256] Nematobothrium filarina, van Bened., on the branchial chamber of the Tunny.
[257] The following conditions represent deviations from this type: (1) In Gasterostomum the oral aperture is situated in the middle of the ventral surface, and occasionally is even nearer to the posterior than to the anterior end. There is no proper oral sucker, but the pharynx is thus termed. (2) A few genera, such as Gasterostomum, Aspidogaster, Diplozoon, etc., have only one intestinal diverticulum, which is undoubtedly to be taken as representing the primitive condition, as it is also often found in the young stages of the Trematoda. (3) The branches of the intestines are curved and united behind (several Tristomidæ and Monostomidæ), while in Polystomum integerrimum (in the bladder of frogs) there are several commissures between the intestinal branches, and in the Schistosomidæ the united intestinal branches proceed as one channel towards the posterior end. (4) The termination of the two intestinal branches is not always on a level; they are therefore of different lengths. (5) When the œsophagus is very long the intestinal branches extend both forward and backward, so that the gut exhibits the form of an H. (6) In the broad and flat species the gut-forks form diverticula mostly externally but also internally; these again may branch (fig. [139]). (7) In a few cases (Nematobothrium, Didymozoon) the intestine completely disappears up to the pharynx.
[258] The following description relates in the main to the Distomata.
[259] The following description relates mainly to the Distomata.
[260] The typical position of the genitalia is subject to many deviations, which are of importance in the differentiation of the genera and families. The following are some few of these deviations: (1) The genital pore remains on the ventral surface, but is situated beside or behind the ventral sucker, or it becomes marginal, and is then found in front of or beside the oral sucker, or at a lateral edge, or, finally, in the centre of the posterior border; the ducts also correspondingly alter their direction. (2) The ovary usually lies in front of the testes, not rarely, however, behind them or between them. (3) The three genital glands mostly lie together close in front of, or behind, the centre of the body; they may be moved far back, and may incidentally become separated one from the other. (4) The vitellarium may be single, in which case it then may lie in the central field. (5) A few forms possess but one, others several or numerous testes. Amongst the ectoparasitic trematodes there are also species with but one testis; but they mostly have several. As a rule, their uterus is short, but the oötype well developed. Special canals (vagina), single or double, are used for copulation, not the uterus. The vitelline ducts also communicate with the intestine through the canalis vitello-intestinalis (fig. [123]).
[261] Monogenea: Trematoda in which the anterior sucker, if present, is double. Development without an intermediate host.
[262] [Recent work (e.g., Goldschmidt, Zool. Anzeiger, xxxiv, p. 482) has shown that the older views regarding the formation of the egg must be modified. In certain species, at any rate, the shell material is formed by the yellow droplets of the yolk glands and not by the so-called shell gland (Mehli’s gland) secretion, which is clear and watery. The function of this secretion accordingly still requires explanation; according to Looss it serves as a covering secretion for the egg-shell proper. It appears also that other granules, the yolk granules as distinct from the shell drop granules, are not always used up during the development of the embryo and hence do not function as yolk, so these also when they exist, and frequently they are wanting, must serve some other purpose, possibly that of imbibing water for the use of the embryo.—J. W. W. S.]
[263] Holostomata: Prostomata with (in addition to the oral and ventral suckers) a third fixation apparatus, generally on a separate part of the body.