Fig. [255].—Diphascon chilenense, Plate, × about 100. (From Plate.) ce, Brain; k, thickening of the epidermis above the mouth; o, egg; oe, oesophagus; p,?salivary glands; ph, pharynx; sa, blood corpuscles; st, stomach.

1a. Sub-genus ECHINISCOIDES differs from the preceding in the number of the claws, the want of definition in the dorsal plates, and in being marine. The single species E. sigismundi, M. Sch., is found amongst algae in the North Sea (Ostend and Heligoland).

2. Genus LYDELLA.[[382]]—The long, thin legs of this genus have two segments, and in other respects approach the Arthropod limb. Marine. Plate suggests the name L. dujardini for the single species known.

3. Genus MACROBIOTUS has a pigmented epidermis, but eye-spots may be present or absent. The eggs are laid one at a time, or many leave the body at once. They are either quite free or enclosed in a cast-off cuticle. The genus is divided into many species and shows signs of disruption. They mostly live amongst moss; but M. macronyx, Doy., is said to live in fresh water. The following species are recorded from North Britain: M. oberhäuseri, Doy.; M. hufelandi, Schultze; M. zetlandicus, Murray; M. intermedius, Plate; M. angusti, Murray; M. annulatus, Murray; M. tuberculatus, Plate; M. sattleri, Richters; M. papillifer, Murray; M. coronifer, Richters; M. crenulatus, Richters; M. harmsworthi, Murray; M. orcadensis, Murray; M. islandicus, Richters; M. dispar, Murray; M. ambiguus, Murray; M. pullari, Murray; M. hastatus, Murray; M. dubius, Murray; M. echinogenitus, Richters; M. ornatus, Richters; M. macronyx?, Doy.

4. Genus DOYERIA.—The teeth of this genus have no support, and the large salivary glands of the foregoing genus are absent; in other respects Doyeria, with the single species Doyeria simplex, Plate, resembles Macrobiotus, and is usually to be found in consort with M. hufelandi, C. Sch.

5. Genus DIPHASCON resembles M. oberhäuseri, Doy., but an oesophagus separates the mouth-tube from the sucking pharynx, and the oral armature is weak. The following species are British, the first named being very cosmopolitan, being found at both Poles, in Chili, Europe, and Asia: D. chilenense, Plate; D. scoticum, Murray; D. bullatum, Murray; D. angustatum, Murray; D. oculatum, Murray; D. alpinum, Murray; D. spitzbergense, Murray.

6. Genus MILNESIUM has a soft oral armature, and the teeth open straight into the mouth. A lens can usually be distinguished in the eyes. Two species have been described, M. tardigradum, Doy., British, and M. alpigenum, Ehrb. Bruce and Richters consider that these two species are identical.

CHAPTER XX
PENTASTOMIDA[[383]]

OCCURRENCE—ECONOMIC IMPORTANCE—STRUCTURE—DEVELOPMENT AND LIFE-HISTORY—SYSTEMATIC

Pentastomids are unpleasant-looking, fluke-like or worm-like animals, which pass their adult lives in the nasal cavities, frontal sinuses, and lungs of flesh-eating animals, such as the Carnivora, Crocodiles, and Snakes; more rarely in Lizards, Birds, or Fishes. From these retreats their eggs or larvae are sneezed out or coughed up, or in some other way expelled from the body of their primary host, and then if they are eaten, as they may well be if they fall on grass, by some vegetable-feeding or omnivorous animal, they undergo a further development. If uneaten the eggs die. When once in the stomach of the second host, the egg-shell is dissolved and a larva emerges (Fig. [260], p. 494), which bores through the stomach-wall and comes to rest in a cyst in some of the neighbouring viscera. Here, with occasional wanderings which may prove fatal to the host, it matures, and should the second host be eaten by one of the first, the encysted form escapes, makes its way to the nasal chambers or lungs, and attaching itself by means of its two pairs of hooks, comes to rest on some surface capable of affording nutriment. Having once taken up its position the female seldom moves, but the males, which are smaller than the females, are more active. They move about in search of a mate. Further, should the host die, both sexes, after the manner of parasites, attempt to leave the body. Like most animals who live entirely in the dark they develop no pigment, and have a whitish, blanched appearance.