The last species of Human En tozoon which remains to be noticed is the Ascaris vermieularis (fig. 7 3), a small worm, also noticed by !lip pocmtes under the name of cr.crxcceK, and claiming the attention of all phy sicians since his time, as one of the most troublesome parasites of chil dren, and occasionally of adults ; in both of whom it infests the larger.
intestines, especially the rectum.
The size of the Ascaris vermicularis varies according to the sex ; the males rarely equal two lines in length ; the females attain to five lines (*fig. 73.) They are proportionally slen der, white, and highly elastic. The head is obtuse, and presents, ac cording to the repeated observa tions of the experienced Rudolphi, the three valvular papilla' charac teristic of the genus Ascaris ; but other Ilelminthologists, who have failed in detecting this organization, refer the species to the genus aryuris. Besides the papilla' the head presents a lateral, semi-obo vate membrane on each side, the broader end being anterior. The body soon begins to grow smaller, and gradually diminishes to a su bulate straight extremity in the female. In the male the posterior extremity is thicker, and is spirally inflected and terminates obtusely ; the head is narrower than in the female.
In the following tabular arrange ment of the internal parasites of the Human body, they are disposed in the classes to which they appear respectively to belong according to their organization.
cies of the Sterelmintha in which a distinct external tegumentary covering can be demon strated. In the Cystic, Cestoid, and most of the Trematode worms, the parenchymatous substance of the body is simply condensed at the surface into a smooth and polished corium of a whitish colour, without any development of pigmental or cuticular layers. The various wrinkles and irregularities, which the super ficies of these Entozoa frequently presents, result from the action of the contractile tissue of the corium : this substance, in the larger Tauter, begins to assume a fibrous disposition, and tears most readily in the longitudinal di rection ; it can be more distinctly demonstrated as a muscular structure in the larger species of Trematoda. By maceration in warm water the rugm of the integument disappear ; the smooth external surface, so well adapted to glide over the irregularities of a mucous mem brane, is then distinctly demonstrated ; and, when magnified, an infinite number of minute pores, variously disposed, are seen perforating the whole surface, especially in the Acantho cephalous worms. It is these pores which, in
the dead worm at least, allow a ready passage to the surrounding fluid into the interstices of the parenchyma, where it sometimes accumu lates so as to swell out the body to three or four times its previous bulk ; and it may be readily supposed, therefore, that the skin here performs some share in the nutrient functions, by absorbing a proportion of the mucous or serous secretions in which the Entozoa are habitually bathed.
In the Acanthocephala the skin, which is but little extensible and friable, is united to the subjacent muscular fibres by means of a whitish spongy tissue which adheres to it most strongly opposite the dorsal and ventral longi tudinal lines or canals. As, however, the skin is with difficulty changed by maceration, while the parts which it surrounds soon go into putrefaction, it can thus be easily separated and demonstrated as a distinct substance. It presents no definite fibrous structure under the microscope, and tears with equal facility in every direction.
In a large Trematode worm, the Distoma clavatum, Rod., which infests the intestines of the Albicore and Bonito, the body is pro tected by a crisp sub-diaphanous cuticle, re sembling in its structure and properties that of the Echinorhynehus.
A similar covering may be demonstrated very readily in the genus Linguatula, among the Calelmintha, and can be separated, but with more difficulty, from the subjacent mus cles in the Ascarides. In the great Round worm (Ascaris lumbricoides) the integu ment is smooth and unctuous, is more exten sible in the longitudinal than the transverse directions, tears with an unequal rupture like a thin layer of transparent horn, and preserves its transparency in solutions of corrosive sub limate, alum, and in alcohol. In this species, in which the digestive canal is completely de veloped, it is worthy of remark that the mi croscope does not demonstrate pores in the cuticle, as in the external covering of the Echinorhynelins and other sterelminthoid worms ; but a series of extremely minute close set parallel transverse lines are brought into view, which are permanent, and depend on the texture of the epidermoid substance itself.