Fig. 206.—Coelomic canals of Glossiphonia complanata. × 10. (After Oka.) a, Dorsal canal containing dorsal blood-vessel; b, ventral canal containing ventral blood-vessel; l, lateral canal; n, nerve-cord.
In transverse sections of leeches it may be seen in successful preparations that the various organs of the body are enclosed in spaces. The funnels of the nephridia open into lacunae which could hardly in any case be regarded as blood spaces, while the blood-vessels themselves with their muscular walls cannot be confounded, at least in the case of the larger trunks, with the spaces not having muscular walls which surround them. Furthermore, it will be pointed out immediately that the reproductive organs are produced on the walls of spaces which are the commencement in the embryo of the reduced coelom of the adult worm. These spaces therefore conform in every particular to the general conditions which have been laid down about the characters of a true coelom. As to the complexity of this system, attention may be directed, to the accompanying diagram (Fig. 206) of the coelom of a segment of Glossiphonia, which has been lately worked out in detail.[[456]] It will be observed that there are four main longitudinal sinuses which are connected by a complicated system of transverse tubes and spaces. In the anterior part of the body, before the point where the intestinal caeca arise, the dorsal and ventral lacunae fuse to form one larger so-called median lacuna. The cavity of this is interrupted, in correspondence with the segmentation of the body, by septa exactly comparable to those of Oligochaeta; but the septa in Glossiphonia are not present at every segment. So far our account of the coelom is chiefly derived from the genus Glossiphonia. In Hirudo, which is an example of the Gnathobdellae, the coelom is still further reduced; the lateral sinuses in them appear to be absent. But on the other hand there is formed a series of cavities in a form of connective tissue which has been termed botryoidal tissue. The cells of this tissue become hollowed out, and form channels which are in communication on the one hand with the remains of the coelom and on the other with the vascular system. This system has certain analogies with the lymphatic vessels of Vertebrates, and, like them, is an intermediary between the body-cavity and the blood. Originally, however, these botryoidal vessels have nothing whatever to do with either the vascular or the coelomic system; their connexion with both is a purely secondary affair, and only appears, comparatively speaking, late in life.
The development of the spaces here spoken of collectively as coelom confirms this interpretation of their nature. In the embryos of Hirudo, Aulastomum, and Nephelis there is a ventral space,[[457]] which includes the nerve-cord. Into this open a series of paired and segmentally-disposed lateral cavities, a pair to each segment. The ventral cavity itself is formed by fusion of the median parts of the lateral cavities. There is here clear evidence of a coelom, developed on a plan fundamentally identical with that of the Oligochaeta in that it is formed as a paired series of chambers corresponding to the segmentation of the body.
Nephridia.—The "segmental organs" or nephridia are seen in their simplest form in such a type as Glossiphonia—the Rhynchobdellae, to which this genus belongs, being indeed in most particulars less specialised than the Gnathobdellae. Here we have a distinct funnel opening freely into the median or ventral coelomic space, which is immediately followed by a rounded swelling termed by Oka[[458]] the capsule; this is filled with cells, in the interstices of which the ductules are situated and meander. There is in this capsule a very strong likeness to the glandular brownish swelling which immediately follows the funnel in the nephridia of certain of the aquatic Oligochaeta, for example the Naids, where, as Vejdovsky has shown, there is a similar "rete mirabile" of the nephridial duct. After the capsule is a single row of cells which are disposed in a complicated coil. These cells are perforated by the duct, which is thus, as in the Oligochaeta, intracellular. In the first set of cells the duct is single, and gives off numerous branchlets into the interior of each cell, a condition which has also been observed in many Oligochaeta. Afterwards the cells are perforated by two, or even three, main ducts, for the duct returns upon itself and traverses the row of cells more than once; there are also branchlets developed from one or other of the main ducts. The terminal part of the nephridium is a short invagination from the exterior, which is lined by cells. There is clearly a close resemblance here with the nephridium of an Oligochaete. The nephridium, however, except for the funnel and the narrow tube immediately following it, does not appear to be ciliated.
Fig. 207.—Nephridium of Hirudo medicinalis. × 10. (After Bourne.) f, Funnel; v, distal vesicle.
There is, however, some difference of opinion as to the portions of the nephridium where there are two ducts in a single cell. Bourne[[459]] thinks that where there are two ducts there are two cells, one lying inside the other, and that there is sometimes also a telescoping of cell within cell where the duct is single. In Hirudo the same writer has described the nephridial funnel, which has lost the simple character of that of Glossiphonia. The funnel is represented by a cabbage-head-like mass (Fig. 207, f) of ciliated cells with no single definite outlet to the exterior as in Glossiphonia. It appears to be an organ which has lost its proper function—a degeneration of the funnel being, as a matter of fact, not unknown in the Oligochaeta, where it may be carried to absolute extinction (Chaetogaster). In Branchellion and Pontobdella the simple metameric arrangement of the nephridia is to some extent lost, owing to the formation of a network continuous from segment to segment. It will be borne in mind that the Oligochaeta are the only other Chaetopods in which such a nephridial network has been stated to exist.
Male Reproductive Organs.—In Hirudo medicinalis there are nine, occasionally ten, pairs of testes, which are round white bodies arranged segmentally, i.e. a pair to each segment. From each arises a slender, somewhat sinuous tube, which enters the common collecting tube of its own side; each of these is much contorted at the upper end, the coiled portion being termed the epididymis. From this they enter a muscular penis which can be protruded. This is the arrangement met with in all leeches, save for the fact that the penis is absent in some; in Glossiphonia (see Fig. 208) this is the case. The number of pairs of testes also varies; and in Nephelis they are no longer arranged metamerically.
Fig. 208.—Nervous system and reproductive organs of Glossiphonia plana. × 2. (After Whitman.) gl, Prostate glands; n, nerve-cord; o, ovary; t, testes.