There are several interesting Vermian genera or groups, which do not fall under any of the thirteen above-mentioned classes, and which can only be briefly touched upon here. They are as follows:
With several species, all minute in size, and marine with the exception of D. sphaerocephalus from brackish water. There is a head (= prostomium), body, and short ventral tail. The head carries two eye-specks and sensory hairs. It is ciliated either uniformly or in two bands. The body is uniformly ciliated (D. vorticoides, D. metameroides\ or the cilia are disposed in rings corresponding to the division of the body into six or seven segments, except on the ventral surface, which is uniformly ciliated. The segmentation of the body is superficial. A nervous system is represented by a ganglion in the prostomium, giving off two anterior and two posterior nerves (D. apatris), or two lateral cords situated in the hypodermis, which disappear in the last segment (D. gigas). The mouth is ventral, and in the first segment of the body (D. gigas); it leads into a pharynx, oesophagus, stomach, and intestine, all ciliated. The anus is dorsal to the tail. Opening just behind and into the mouth is a cavity, which contains a protrusible muscular proboscis, as in Archiannelida. There is a coelome, traversed by strands of connective tissue. An excretory system is present.
In D. gyrociliatus it consists 'of five pairs of intracellular segmental canals, each with its own external aperture, and terminating internally in a flame-cell' (Lang, quoting Meyer, 'Polycladen,' Fauna, etc, des Golfes von Neapel, xi. p. 678). Flame-cells have been detected in D. gigas and D. apatris. In the latter a network of fine canals is said to exist beneath the integument, as well as larger ciliated canals. The larger canals, however, are not present in the head. Korschelt observed one of them to open on the ventral surface near the ovary. The sexes are separate. Males and females are alike in D. gigas and D. vorticoides, dissimilar in D. apatris. The male of the latter is much smaller than the female; its head has a single ring of cilia, and the ventral surface of its body is uniformly ciliated; it has no eyes, no digestive tract; a conical perforated copulatory organ lies within a cavity at the posterior extremity of the non-segmented body. The ovary is single in D. apatris, and is attached to the digestive tract where the stomach passes into the intestine; the ova are set free into the coelome as large female, and small male, ova, surrounded by a substance which swells up when they are laid; the birth opening is ventral, and in front of the anus.
Segmentation in D. apatris is unequal; the gastrula epibolic; and the first stages of the development of the male resemble those of the female. In D. gigas the testis and ovary are Y-shaped masses of cells, situated as in D. apatris. The genital products collect in the coelome, and probably destroy the parents by rupture. Two or four ovaries are stated to exist in other species.
Dinophilus is an interesting genus. It is probably to be regarded as a modified Trochosphere, differing very markedly from the modification which gave origin to the Rotifera.
D. apatris (with lit.), Korschelt, Z. W. Z. xxxvii. 1882. D. gigas, Weldon, Q. J. M. xxvii. (I), 1886.
This group of minute organisms is defined by Ludwig as follows: 'Small vermiform animals, with a well-defined ventral surface. Digestive tract straight, divisible into an anterior muscular, and a posterior cellular, region. Mouth and anus ventral. Body clothed with a cuticula, which bears processes differing in character. Cilia, as a rule, confined to the ventral aspect, rarely covering the region of the head. Posterior end of the body, as a rule, forked. No nervous system recognisable. Hermaphrodite? Reproduction by means of summer and winter ova; no metamorphosis.' Eyes may be present. The female aperture lies dorsally in front of the forked end of the body; the ova are set free into the coelome. The distinction between summer and winter ova appears to be known only in Ichthydium (Chaetonotus) larus. The freshwater genera are Ichthy-dium (which, according to Ludwig, includes Chaetonotus), Chaetura, Cephalidium, Dasydites, and the marine genera Turbanella and Hemidasys.
Ludwig, 'Gasterotricha,' Z. W. Z. xxvi. 1876. Chaetonotus, Butschli, ibid. Hemidasys, Claparede, A. Sc. N. (5), viii. 1867.
Marine. Minute, with a cylindrical body and flattened ventral aspect. The body is segmented into eleven to twelve chitinoid rings, of which the first or head is invaginable, and is armed at its anterior margin with four rows of recurved hooks j the second is provided with twelve longitudinal chitinoid ribs, and closes over the first when invaginated; the third and fourth are entire, and the remainder are divided on the ventral aspect, by a median and two lateral fissures, into a large dorsal and two smaller ventral plates. The terminal or anal ring is usually forked. Most of the rings carry spines, - the last usually two of great length, more seldom one. A nervous system is represented by a right and left band, which unite anteriorly (Greeff); and two to eight red eye-specks lie upon the bands. Reinhardt, however, states that he could not find the nervous bands, but found instead four glandular caeca opening into the proboscis. The mouth is anterior and terminal, and leads into an eversible proboscis, armed at its apex with six to eight two-jointed spines; there is a muscular oesophagus, with a circlet of small spines at its commencement, an intestine, and terminal anus.
Excretory organs are represented by a pair of sacs in the ninth segment; they end in ciliated ducts, which open separately on the latero-dorsal aspect of the tenth segment. The sexes are separate. The testes and ovaries are paired and saccular; they open terminally. Greeff considered the animal to be viviparous, but Reinhardt states that he mistook the testes with their contents for ovaries. There is a single genus, Echinoderes, with several species.
Reinhardt, Z. A. iv. 1881; Greeff, A, N. 35 (I), 1869; Panceri, Atti Accad. delle Scienze, Naples, vii. 1878, No. 10, p. 4.
Minute, marine, vermiform, with the body contracting -both in front and behind. The head is somewhat globular; the body is girt by a series of rings, varying in number in the different species. There are four setae on the head, and a pair on more or fewer of the body-rings, some dorsal and some ventral; they are capable of independent motion. There are two eye-specks, but no nervous system is known. The mouth is anterior and terminal; there is a muscular oesophagus, a straight intestine, and an anus on the dorsal aspect, according to Greeff. The sexes are separate; the testis and ovary are single and saccular; the former opens into the anus, and has two chitinoid copulatory spiculae; the latter opens anteriorly to the anus on the same aspect (GreerT). There is a single genus Desmoscolex, with several species. The genus Trichoderma (GreerT), which has no setae, but is covered with long hair-like processes, is perhaps related to Desmoscolex. The male has two spiculae. Both genera are in some respects like the Nematoda.
Greeff, op. cit. supra, and Panceri, op. cit. p. 2 and p. 7; Reinhardt, Z. A. iv. 1881, p. 591.
Minute; partly marine, partly freshwater. There are three genera, Chaetosoma, Tristicochaeta, and Rhabdogaster. The first-named has an oval head, and a body pointed posteriorly. The head has a double row of moveable hooks, arranged in a semi-circle; and it is covered, as also the body, with a number of fine hairs. Locomotion is effected by a double row of knobbed rods anterior to the anus. The mouth is anterior and terminal; there is a muscular oesophagus, an intestine, and a rectum, with a ventral anus. The sexes are separate. The testis is a single sac; the vas deferens opens with the anus, and has two chitinoid copulatory spicules. There are two ovaries, with a single vagina opening ventrally about the middle of the body. Tristicochaeta (Panceri) resemble Chaetosoma very closely, but has three rows of locomotor rods. Rhabdogaster has the head not marked off from the body, but it is somewhat dilated, like the genital region. The hairs are restricted to the dorsum. The locomotor rods are very slender and hooked, situated so far anteriorly that the vagina opens among them.
Chaetosoma and Rhabdogaster, Metschnikoff, Z. W. Z. xvii. 1867. Tristico-chaeta, Panceri, op. cit. supra, p. 7.