(i.) CHAETOSOMATIDAE.—This family includes three genera: _Chaetosoma_, _Rhabdogaster_, and _Tristicochaeta_. According to Metschnikoff,[198] although they are not true Nematodes, they have a great likeness to the group. He distinguishes them from the swimming members of the group as "creeping Nematoda." _Chaetosoma_, of which two species are known, _C. ophicephalum_ and _C. claparedii_, has a head distinct from the body (Fig. 79). The mouth is at the anterior end, surrounded by a double semicircle of movable spicules; the whole body is covered by fine hairs, and on the ventral surface, just in front of the anus, is a double row of about fifteen cylindrical projections, by whose agency the animal creeps. The female _C. claparedii_ is 1.5 mm. long, the male 1.14 mm. They were found creeping about on sea-weeds in the neighbourhood of Salerno. [Illustration: FIG. 79.—Mature female of _Chaetosoma claparedii_ Metschni., × 57. (From Metschnikoff.) _a_, Oesophagus; _b_, intestine; _c_, anus; _d_, ovary; _e_, generative pore; _f_, ventral bristles.] The genus _Tristicochaeta_[199] differs from the foregoing in having three rows of locomotor projections instead of two. [Illustration: FIG. 80.—_Tristicochaeta inarimense_ Panceri, in one of its most usual positions, showing the triple row of ventral bristles, × 100. (From Panceri.)] _Rhabdogaster_ has no head distinct from the body, though the anterior part of the body is swollen. A second swelling occurs, as is also the case with _Chaetosoma_, in the region of the opening of the genital ducts. The female in _Rh. cygnoides_ attains a length of 0.36 mm. In this genus the hairs are confined to {159}the dorsal middle line. The locomotor projections are hooked, and are much finer than those of _Chaetosoma_, and they are situated farther forward than in the last-named genus. _Rhabdogaster_ occurs in the same surroundings as _Chaetosoma_. _Ch. ophicephalum_ is recorded from the English Channel. (ii.) DESMOSCOLECIDAE.—The members of this family are minute, and are characterised by the presence of well-marked ridges which surround the body and give it an appearance of segmentation. The head, which is somewhat swollen, bears four bristles, and single pairs are borne by a certain number of the ridges, some on the dorsal and some on the ventral surface. These hairs can be moved independently of one another. Two red eye-spots are described between the fourth and fifth rings. The sexes are distinct, and the internal organs generally have a marked resemblance to those of the true Nematoda. The _Desmoscolecidae_ move by looping their bodies after the manner of the Geometrid caterpillars, as well as by creeping with their bristles. The genus contains numerous species[200]: _D. minutus_ Clap. (English Channel), _D. nematoides_ Greef, _D. adelphus_ Greef, _D. chaetogaster_ Greef, _D. elongatus_ Panceri, and _D. lanuginosa_ Panceri. They are exclusively marine.