402 LEECH
mouth of the cup Ranke figures retinal cones (Glaskörper- kugeln), which are arranged like nerve end-organs in a mosaic, on a flat extension of the optic ganglion. These cones are very like those of the vertebrate eye, consisting of a somewhat rounded granular body, connected at the base with a nerve-filament, and having a clear, stiff, rod-like projection on its outer part. Dr Gunn has been unable to see these cones or the termination of the optic nerve. The wall of the clear cell is very thick, and the "nucleus" is generally seen to be an inward projection of this wall ending in a knob-like enlargement. Where it appears free, that is probably due to the plane of section, the side or end of the knob being severed from its connexion. Besides the cells having this inward projection of the wall there are others containing highly refractive spherules like oil globules. Ranke observes how little these "optic cups" differ from the touch or taste organs scattered on the snout and sides of the animal, and he is of opinion that they probably serve equally for the three kinds of sensory per ception (sight, taste, and touch). If Ranke's account is correct, and if the cone-mosaic situated at the mouth of the cup be directly stimulated by the rays of light, it is difficult to account for the function of the large clear cells, and more especially the pigment around. From the position of the pigment it cannot serve for the isolation of Ranke's elements, and it can hardly be required for the prevention of the confusion of images. Yet by its presence the eye of the leech is distinguished from the adjacent and very similar touch-organs. Dr Gunn is of opinion that the light acts on the pigment, and develops some form of energy which affects the contents of the cell, whence a stimulus is communicated to the nerve. Unfortunately a nervous connexion with these cells has not been found.
The three anterior pairs of ventral ganglia (b) coalesce into a single mass, and in the same way the last large ganglion (b') is composed of seven. The ordinary ventral ganglia give off two branches on each side, one of which has a small ganglion developed on it. The penultimate ganglion sends off only a single branch on each side, while the last gives off from seven to nine for the supply of the posterior sucker. The nerve-cells, as usual in these ganglia, are chiefly external, and the fibrous region internal, while the whole is surrounded by a neurilemma. This system has been the subject of many elaborate researches, amongst which those of Leydig and Hoffmann are conspicu ous. A sympathetic or azygos nerve discovered by Brandt runs along the ventral surface of the digestive tract. In development it is found that in many leeches the long cords are originally separate, but afterwards come close together so as to resemble a single connecting cord.
Circulation.
The circulatory system presents a median dorsal, a median ventral, and two large lateral longitudinal trunks, all anastomosing with each other, and giving off numerous branches to the muscular layer of the mesoderm and various internal organs. The median sinus in the head surrounds the ganglia and oisophageal ring. It has a ventral develop ment in the rest of the body, where it encloses the alimentary canal and the gangliated nerve-cord. The blood-vessels have a well marked systole and diastole from eight to ten times per minute. The fluid is red, and devoid of corpuscles. Old observers noticed the finely reticulated condition of the integuments when the vessels were injected, but, as formerly noticed, vessels could not be seen in the hypoderm proper. The active to and fro waving movements of leeches in the water when attached by the posterior sucker are probably connected with cutaneous respiration.
Segmental organs.
No part of the leech has caused more discussion than the series of seventeen pairs of segmental organs (e, e') which occur in a line external to the testes, and alternating in position with them. Some considered them respiratory, others excretory, while a few connected them with the reproductive system. They consist of a muscular saccate ciliated organ which communicates with the exterior near the posterior part of each primary segment, and externally of a loop-shaped gland, labyrinthine in structure, one end of which opens into the former sac, while a cæcal process is prolonged on each of the testes in their region. In minute structure it has been found that the cells which constitute the gland are all penetrated by ductules, which, however, do not communicate with the large duct in the axis of all the lobes (Bourne). The gland is surrounded by an elaborate plexus of blood-vessels. These organs are in the embryo preceded, in the posterior region of the body, by three pairs of looped canals, which disappear before the permanent ones are developed.
Reproduction.
The leech is hermaphrodite, but congress of different individuals is necessary for reproduction, and thereafter spermatophores, which have a special covering, are found in the respective vaginæ. The male organs consist of an intromittent apparatus (h) with a muscu lar and glandular basal structure (g), and a duct (f) on each side from the vesicida seminalis. The latter has a vas deferens connect ing it with the nine globular testes (d, d', d", &c.) ranged along each side of the body, one of which is displaced outward at d". The intromittent organ reaches the exterior at the junction of the first and second sixth of the body (between the twenty-fourth and twenty-fifth rings). The female apparatus is placed in the segment between the seminal vesicles and the first testis, four annuli inter vening between the respective sexual apertures. The external opening of this system (between the twenty-ninth and thirtieth rings) leads into an oval sac (j), the vagina – furnished with thick muscular walls. A coiled oviduct passes from its apex through glandular tissue, which probably secretes the albuminous matter surrounding the eggs, and divides into branches, one leading to each ovary (i). In Hæmopis the ovaries form a coiled filament, and on this the ovarian germs are budded. The ova are connected with the filament by a thin envelope which is drawn out into a stalk. There is no cord in Nephelis, but the ovarian germs form groups of cells.
Three or four days after congress the leech may be observed to be contracted above and below the genital apertures, and an abundant secretion is poured out so as to surround this region of the body as in the Nemerteans. Into this investment the contents of the female organs and their opaline gelatinous envelopment are forced. The animal elongates the anterior part of its body, withdraws its head, and the structure just mentioned slips off as a cocoon containing from five to eighteen ova, and frequently showing slight elevations at the points through which the body passed. The cocoons are deposited in cavities in the mud during the summer and autumn, and some seem also to deposit them during the winter. The ovoid cocoons consist of a network of spongy fibres, and indeed have been mistaken for a sponge. The older authors considered the leech viviparous until Noble and Rawlins Johnson observed the foregoing phenomena.
Leech breeding.
There is little difficulty in rearing leeches in confinement if a proper method is followed, and accordingly various leech tanks and ponds have been constructed. One of the largest schemes of the kind is a leech farm of 13 acres near Newton, Long Island, U.S. The breeding ponds are in oblongs, each of 1½ acres in extent, and 3 feet or more in depth. The bottom is composed of clay, and the margins of peat. The cocoons are deposited in the soft peat from June onward, the chief enemies being musk-rats, water-rats, and water-shrews, which dig the cocoons out of the peat. The adult leeches are fed every six months on fresh blood placed in linen bags suspended in the water. It is also the opinion of some that leeches which have been filled with blood make good breeders.
Development.
In regard to the development of the Gnathobdellidæ, Nephelis, perhaps, has been more completely worked out than Hirudo (though the observations of Weber, Leuckart, Robin, and others on the latter are important), and, as the former very much resembles the latter, except in the presence of cilia in the embryo anteriorly, a brief notice of it will suffice. Butschli describes the usual divisions of the eggs, which need not be given in detail, especially as an excellent summary is to be found in Balfour's Comparative Embryology. According to these authors the cells which constitute the epiblast give origin to others which form the hypoblast and vitelline spheres. Two patches of epiblast gradually spread over the vitelline spheres. Then the hypoblast cells increase and fill up a space bounded behind by three vitelline spheres and in front by the epiblast of the anterior end. At the sides of the hypoblast the mesoblast has become established, probably as two lateral bands. The hypoblast cells range themselves round a central cavity, increase, and become filled with food-yolk. The mouth and thick-walled œsophagus are then developed, probably by epiblastic invagination.
