Bilaterally symmetrical animals, without limbs and with a body cavity in which the gut or other organs float. They are generally cylindrical.
Class. NEMATODA.
Nemathelminthes with an alimentary canal.
Nematodes are as a rule elongated round worms of a filiform or fusiform shape; their length varies according to the species from about 1 mm. to 40 to 80 cm. The outer surface of the body is smooth or annulated, and at certain points provided with papillæ, occasionally also with bristles and alar appendages. The anterior end carrying the oral aperture is usually rather slender, occasionally quite thin; the posterior end is pointed or rounded; the anus, as a rule, lies somewhat in front of the posterior extremity. The sexes are almost always separate, and the male can as a rule be easily distinguished from the female because the former is smaller and more slender, its posterior extremity is often spiral or incurved, or carries an alar appendage, whereas the female is larger and thicker, and its posterior extremity is straight. In the male the genitalia open into the anus; the sexual orifice of the female opens ventrally along the median line in the anterior half of the body, in the middle, or a little further back. Both sexes, moreover, have an orifice, the excretory pore, which is situated ventrally in the median line and about the level of the œsophageal nerve ring.
In large species, even with the naked eye, two lighter transparent bands—the lateral lines—may be distinguished; they run along the sides of the body from the anterior to the posterior end, while two other bands, the median lines, running along the ventral and dorsal mid-lines, are less evident; in exceptional cases there are also four sub-median lines. These bands or lines are inward projections of the ectoderm, and in them lie the nerves and excretory vessels (fig. 260).
Some Nematodes live free in fresh or salt water, in soil, mud or decaying vegetable matter, others parasitically in the most various organs of animals, frequently also in plants.
Anatomy of the Nematodes.
All the Nematodes are covered by (1) a CUTICLE, which in the small species is thin and delicate, while in the larger species it is thickened, and may consist of several layers of complicated structure. Canalicular pores do not occur. According to general opinion, which is confirmed by the history of development, the cuticle is a product of (2) the EPITHELIUM or ectoderm that had formerly existed or is still found beneath it; in young specimens and small species it is perceptible, but in older worms it frequently alters so considerably that not only do the borders of the cells disappear,[297] but a fine fibrous differentiation appears in their cytoplasm. The matrix or ectoderm then has the appearance of an ectodermal syncytium permeated by fibres and strewn with nuclei, so that it is hardly distinguishable from the tissue of (3) the CUTIS, which is always present, though developed to a varying degree. Both layers, matrix and cutis, project internally as ridges and form the lateral lines, while the less marked median lines are produced apparently only by the ectoderm (fig. 260).
