ANNELIDA - HIRUDINEA The Hirudinea or, popularly, leeches, are hermaphrodite Annelids provided with a sucker at both ends. There are no parapodia or chaetae, except in Acanthobdella, a form which is assigned by some authorities to the Oligochaeta. There are no appendages except branchiae, which occur in a few species. The number of segments is constantly thirty-four. The coelom is reduced to a series of intercommunicating spaces except in Acanthobdella, where it consists of a series of segmental cham bers as in the Oligochaeta. A clitellum is present and the testes are usually numerous and continuous with the sperm-ducts. There is a single pair of ovaries continuous with the oviducts and the sexual pores are median and single. The nephridia are paired, always with an intracellular lumen and often with closed funnels; rarely they form a network from segment to segment. Eyes are usually present and the nervous system lies in the coelom. The Hirudinea are aquatic, terrestrial and marine ; they are carnivor ous and parasitic.
Annulation.—The limits of the segments in the Hirudinea are obscured by the presence of annuli or rings, which in the fully developed segment of the middle body may vary in number from two to fourteen. The 34 segments correspond to and are innervated from 34 ganglia in the nervous system ; and the num ber of segments and their limits is determined by the number and position of the ganglia. The ring in which the sensillae or segmental sense organs externally and the ganglia internally are located is normally the central ring of the segment.
The body may be divided into five regions. The cephalic region, consisting of six segments, the majority of which are composed of a single ring, includes the an terior sucker, the mouth, the eyes and the jaws, when present. The anterior sucker may be discoid as in the majority of the Icthyob dellidae or spoon-shaped as in most Arhynchobdellidae. The eyes are of ten complex, consisting of an optic nerve and ganglion, a sheath of visual cells containing a vitreous body, the whole sur rounded by a dense cup of pig ment. The second region or pre clitellum contains the pharynx and anterior nephridia, and is marked by the intermediate con dition of development of the seg ments. The middle region, fur ther divided into the clitellum and post-clitellum, contains the main part of the organs of the reproductive, digestive and ex cretory systems. The clitellum, which usually extends over three complete segments, consists of a thick layer of unicellular glands. The postclitellum is followed b;' the anal region, in which the segments are again incomplete. This leads to the posterior sucker, composed of segments 28 to 34, disc-like and directed ventrally except in the burrowing Lum bricobdella; it constitutes the caudal region.
The sensillae or segmental tactile organs are found on the sensory ring of each segment. They appear as round white patches on the skin, and there are usually four to six pairs to each ring; they are provided with sensory hairs and are retractile.
The musculature is very complex; besides the main oblique, circular and longitudinal layers, of which the last are the thickest, there are vertical and transverse sheets of fibres.
Coelorn.—The most remarkable feature of leech morphology is the coelom or body-cavity. In Acanthobdella its condition re sembles that found in the Oligochaeta, but in other leeches it is largely filled by the growth of connective tissue of various kinds and reduced to a series of canals or lacunae connected by an elaborate system of delicate branches. In the Glossip/ionidae there are five main longitudinal lacunae; hypodermal lacunae are also present in the form of fine canals running round the body just beneath the skin. By bringing the contained coelomic fluid close to the air they assist in respiration. Most of the Icthyob dellidae possess strongly contractile lateral lacunae, and certain members of the family have pulsating vesicles beneath the skin, resembling cutaneous sacs which are rhythmically inflated and de flated. These are filled with coelomic fluid from a branch of the segmental lacunae encircling the body, and the contraction of the muscular walls of the vesicles drives the fluid into the lateral lacunae; the vesicles are provided with a valve formed by an invagination of the wall, which opens and closes as they pulsate.
The Rhynchobdellidae have a closed vascular system in addi tion to the lacuna system, and in the Arhynchobdellidae, in which the obliteration of the coelom has advanced still farther, the lacuna system, wholly or in part, takes the place of the vascular system of the former family; in the Arhynchobdellidae the coelom is invaded by the so-called botryoidal tissue, consisting of curious pigmented and excavated cells, the precise function of which is still not fully understood.
Reproductive System.—The leeches are hermaphrodite with paired male and female organs. The testes are segmentally ar ranged, except in certain Arhynchobdellidae in which they are subdivided into numerous small ovoid bodies. A small segmental duct leads to the vas de f erens on each side, which enlarges an teriorly to form an epididymis or sperm-vesicle ; this leads into the dcctus ejaculatorius, which finally empties into the common eversible male organ. The female organs consist essentially of two large ovisacs, containing the ovaries and leading to the ex terior by a common female aperture.
The sperm is packed in spermatophores, in which it is con veyed from one individual to another. Copulation occurs in the Hirudidae by means of the eversible male organ, but in the other families impregnation is normally hypodermic. One leech plants a spermatophore upon the body of another leech ; the sperm escapes and makes its way through the tissues into the ovisacs where fertilization takes place. In many groups the spermato phores have to be planted on a definite area of the body, beneath which lies the special conductive tissue through which the sperm passes.
Nepliridia.—The convoluted nephridia are formed of rows of tubular cells set end to end like drain-pipes. Externally they open by a vesicle on the ventral surface of the segment, and the internal funnel communicates with the lacuna system.
Pliylogeny and Classification.—There is much evidence to show that the leeches are derived from the Oligochaeta. Apart from Acanthobdella, to whose systematic position reference has already been made, the leeches are divided into two suborders:— the Rliynchobdellidae, marine and freshwater leeches with colour less blood, with an exsertile proboscis, without jaws; and the Arhynchobdellidae or Gn athobdellidae, freshwater and terrestrial leeches with no proboscis, typically with jaws, with red blood.
Development.—The eggs are laid in cocoons formed from glands in the clitellum ; segmentation is unequal and in the Arhynchobdellidae the young hatch early and swim about inside the cocoon, feeding on the contained albumen. Af ter leaving the cocoon the young of ten attach themselves to the ventral surface of their mother.
Distribution.—The Continental leeches are limited in range, both vertically and horizontally, by temperature alone. Below 40°F they cease to be active, and they are not found in the polar regions or above certain altitudes in the mountain ranges. The marine leeches, however, are carried by their piscine hosts far into the polar seas.
Economics and Ecology.—A brief sketch of the economic re lations of leeches is given in the article LEECH. For further in formation and for an account of their habits, with special refer ence to tropical forms, the reader is referred to Harding and Moore's "Hirudinea," Fauna of British India (London, 1927).