Multisegmental Vermes, in which the original somites are masked by the development of secondary annuli. The posterior somites are always fused to form a terminal sucker. The original coelome is obliterated for the most part by a growth of connective tissue, and is represented by a series of sinuses and channels with which the true blood-system communicates. The mouth is anterior and ventral, and the anterior extremity of the body in which it lies is. often marked off as a more or less efficient sucker. The anus is dorsal and anterior to the posterior sucker. Hermaphrodite. The genital openings are median and ventral; the male in front of the female. For the most part aquatic, parasitic, and blood-sucking.
The ventral surface of the body is flattened, the dorsal convex. The animal creeps by alternately attaching and detaching the anterior and posterior extremities or suckers, and swims by undulations of the body. The body is segmented. The Hirudinidae possess twenty-six somites, and these somites in them as well as in all other Leeches are annulated. A variable number of annuli, or rings, belong to a somite, e.g. in Hirudo five, Pontobdella four, Branchellioit three. In the cephalic and posterior regions of the body, the number in the somite is less, and may most anteriorly be reduced to a single ring. A first annulus in the middle region of the body contains a nerve ganglion and a nephridial funnel, and it may be marked externally by peculiar arrangements of colour, of integumental papillae, or by the presence of segmental sensory papillae. The anterior extremity of the body is more or less expanded, and its annulation lost in the Leeches with a pharyngeal proboscis (Rhynchobdellidae), but whether expanded or not, it always acts as a sucker. The two or three somites which contain the genital openings constitute the clitellum, and at the reproductive season secrete on their surface the cocoon in which the ova are contained.
These genital somites are the ninth, tenth, and eleventh in Hirudo (Whitman); the sixth and seventh post-oral of Pontobdella (A. G. Bourne). The marine Branchellion Torpedinis has a pair of lateral folia-ceous expansions to each annulus of the middle and posterior regions of the body. Similar folds are found in Lophobdella from the mouth of the Crocodile, etc. Acanthobdella has setae.
A delicate elastic cuticle, which is moulted and renewed at intervals, covers the body. It is pierced by the orifices of the cutaneous glands, nephridia, etc. The epidermis consists of a single layer of cells, between which there penetrate in all Gnathobdellidae and some Rhynchobdellidae processes of pigmented connective tissue cells together with blood capillaries. There are numerous integumental unicellular glands of various kinds; mucous glands which either retain their position in the epidermis, or when large intrude into the dermis; prostomial glands in Gnathobdellidae, opening round the buccal cavity; salivary glands opening into the pharynx; and clitellar glands opening on the clitellar somites; all deeply placed in the dermis and provided with long ductules. The body-walls are composed of an external circular and an internal longitudinal layer of muscles, with an interposed diagonal layer in some instances. These layers are imbedded in a plentiful connective tissue, which forms also a superficial sub-epidermic coat, and Consists of a variably developed jelly-like matrix containing cells of various kinds, among them branched connective tissue and pigment cells (see p. 215). Dorso-ventral and radial muscle bundles take the place of the septa of Chaetopoda.
The nervous system consists of a paired supra-oesophageal ganglion, connected by a very narrow oesophageal commissural ring with the first ganglion of the ventral chain, which consists in most Leeches of twenty-three ganglia, united by longitudinal commissures. The first or sub-oesophageal and the terminal ganglion are formed by the fusion of several embryonic ganglia (p. 219-20). The ganglionic cells have a tendency to form projecting masses on the surface of the ganglia. In Hirudo there are three minute ganglia connected one with each of the three jaws. A sympathetic nerve is in relation with the stomach. A variable number of eyes are present on the anterior cephalic annuli and of Piscicola on the posterior sucker. They are entirely absent in some terrestrial forms (Typlobdella, Cylicobdella, Macrobdelld). Eye-like organs, but devoid of pigment and known as 'segmental papillae/ are found on the first annulus of each somite, and sometimes on the posterior sucker in Gnathobdellidae. They occur in Clepsine, and it is possible that they exist also in other Rhynchobdellidae. Sense organs or goblet-shaped bodies, with a structure closely resembling eyes, are scattered in large numbers over the cephalic annuli of some Leeches, and on the lips 1
1According to Whitman (Leeches of Japan, Q. J. M. xxvi. pp. 396-410), the goblet organs of the lip consist of a clump of elongated hypodermis cells, and of a nerve distributed to the base of the clump in which there are what appear to be ganglion cells. Clear, nucleated, and vacuolated cells are found scattered along the nerves going to these organs. The segmental organs and the scattered sense-bulbs of the head, on the contrary, have a number of vacuolated cells grouped round the clump
The alimentary canal consists of a pharynx, oesophagus, proven-triculus, digestive stomach, intestine, and rectum. The pharynx has muscular walls, with the muscles arranged as radiating and circular fibres, very largely developed in the Rhynchobdellidae, where it and the part of the body which lodges it, constitute the retractile proboscis characteristic of the group. In the typical Gnatkobdellidae the radiating muscles of the pharynx form three prominent ridges which have their cuticle produced into the three well-known toothed jaws. The Australian Land Leeches (Geobdella, Whitman) possess only the two lateral jaws. In other cases, e. g. Trochetay the jaws are quite rudimentary, or even, as in Leptostoma edentulum, lost altogether1. There is an oesophageal tube, short in Hirudo, longer in most other Leeches, thrown into folds in Clepsine when the proboscis is retracted. The proventriculus is a feebly muscular tube with lateral paired caeca in most Hirudinea, the number of pairs not being the same in all instances 2. The last pair bends backwards parallel to the axis of the body. Aidostoma and one or two others possess this last pair alone. Pontobdella has a single median ventral caecum underlying the stomach, whilst Trocheta and Nephelis are devoid of caeca altogether.