Fam. 1. Gnathobdellidae.—Pharynx with three denticulate jaws.
This family as well as the next is terrestrial or fresh-water in habit. It contains a number of generic types, including the medicinal leech, Hirudo medicinalis, and the horseleech, Haemopis (Aulastomum) gulo. The former can be distinguished from the latter by its power of contracting itself into an oval olive-shaped form, which power is not possessed by the horseleech; the latter has, moreover, only two caeca, while the common leech has ten pairs of these appendages of the intestine. The genus Limnatis is called after the Greek word λιμνῆτις, which Theocritus applied to the leech. It is found in the Nile, and caused serious inconvenience to the army of Napoleon. His soldiers in drinking at pools sucked up the small leeches not thicker than a horse's hair, whose presence in the hinder part of the mouth cavity produced divers objectionable results, such as spitting of blood and hindered respiration.
Fam. 2. Herpobdellidae.—Pharynx without denticulate jaws, with three unarmed chitinous plates.
A characteristic genus of this family is Trocheta, which is so common at the Zoological Society's Gardens and in the Regent's Park, and which has been met with in other places near London; it is in this country an introduced species, but is found in many parts of the continent. It is a land-leech, and lives upon earthworms.
The genus Haemadipsa, which M. Blanchard places in a special sub-family, contains a number of species which are for the most part land-leeches. Land-leeches occur in many parts of the world, but chiefly in the tropics—in India, Ceylon, Java, South America, etc. They lie in wait for their prey, upon the ground as a rule; but they may ascend herbs and shrubs to gain a better outlook when they are aware of an approaching footstep. A vivid account of the ferocity of these tiny Annelids in Ceylon can be read in Sir J. E. Tennent's Natural History of Ceylon. They have been said to be so pugnacious and so poisonous that persons surprised in their sleep by the pests have succumbed to their united efforts. A whole battalion of English soldiers decamped on one occasion from a wood which was overflowing with land-leeches. The familiar misquotation "lethalis hirudo" might well be applied to this species. Professor Whitman has written much upon the habits of the land-leech of Japan (Haemadipsa japonica), which bites so softly that its presence cannot be detected except for the stream of blood which trickles from the wound. While it is feeding it emits from the pores of the nephridia a clear fluid, which, as it appears, is used to keep the skin moist; when unduly dried the same phenomenon occurs. It is curious that in this and other leeches the nephridia should play a part which in the earthworm is played by the dorsal pores; in both animals the glands of the skin are also concerned with the same duty.
The purely aquatic leeches swim by undulations, and also crawl by the help of the two suckers, like a "Geometer" caterpillar. But when a land-leech is dropped into the water it at once sinks to the bottom and crawls out; it does not swim, but can survive immersion for a long period. In this it resembles the earthworms, which can also survive a prolonged immersion, and even in the case of some are indifferent to the medium, land or water, in which they live; the land-leech, however, is entirely dependent upon damp surroundings; a dry air is fatal to it. The land-leech of Japan leaves a slimy trail behind it as it crawls, in this respect recalling the land Planarian Bipalium kewense.
GEPHYREA AND PHORONIS
BY
ARTHUR E. SHIPLEY, M.A.
Fellow and Tutor of Christ's College, Cambridge