Fig. 204.—Branchellion torpedinis. (From the "Règne Animal.") ×1.
Some Hirudinea are furnished with external branchiae; this is the case with Branchellion, in which genus the branchiae (Fig. 204) have an arborescent form; in Cystibranchus there are a series of paired simple vesicles which take the place of these more complicated respiratory organs of Branchellion. The Hirudinea do not, save for one exception (Acanthobdella), possess chaetae; but it must be borne in mind that the Discodrilidae and the genus Anachaeta among the Oligochaeta are in the same condition. In Acanthobdella[[454]] there are two pairs of chaetae upon each side of the anterior five segments of the body. According to the figure which Grube, the original describer of the genus, gives of these chaetae, the part implanted in the body is straight, while the part extending freely beyond the body is sharply hooked.
The body of the leeches is never ciliated externally; there is, as in the higher Oligochaeta, a cuticle secreted by the underlying epidermis. The Hirudinea have, like the Oligochaeta, a clitellum which, as in some of the lower members of that group, is limited to a very few segments in the immediate neighbourhood of the generative openings. It occupies in Hirudo segments 9, 10, and 11. The epidermis gives rise to many unicellular glands which either remain in situ or get moved to a deeper position. In this the leeches exactly resemble the earthworms. There is generally a great deal of connective tissue in the body-wall. The muscles consist of circular, longitudinal, and radial series. The individual fibres have the same structure as those of the Oligochaeta, consisting of a soft and undifferentiated core, round which is a radially-striated sheath of contractile substance.
Alimentary Canal.—The leeches are divided into the Rhynchobdellae, which have a proboscis but no jaws, and the Gnathobdellae, which possess a series of jaws but have no proboscis. But the division is not a hard and fast one, for we have Whitman's genus Leptostoma, which should belong to the jawed division, but which has quite rudimentary jaws without the sharp denticles so characteristic of Hirudo. The pharynx is furnished with salivary glands. The oesophagus is followed by the proventriculus, which has a varying number of pairs of caeca; then comes the intestine and the rectum. The anus is, as a general rule, placed dorsally to the sucker, but there are a few rare exceptions where the anus is within the sucker. The caeca are totally absent in Trocheta.
Vascular System.—As will be seen from Fig. 205, the vascular system of the Hirudinea is constructed on a plan which closely resembles that of the Oligochaeta. The diagram represents Glossiphonia, one of the Rhynchobdellae, the group which comes nearer to the Oligochaeta in many particulars than the Gnathobdellae. We can recognise a dorsal and a ventral vessel, which are united in the anterior part of the body by three perioesophageal rings, the elongation of which, particularly of the last pair (v), from before backwards is very marked. In the region of the sucker the dorsal and ventral vessels are united by fourteen shorter loops, the number of which has an interesting relation to the number of segments out of which this portion of the body is possibly formed. It will be observed also that the dorsal vessel is double in this region, a condition which obtains along its whole length in Branchellion—a repetition of what has been described in more than one species of Oligochaete. In the region of the last pair of digestive caeca the dorsal vessel has appended to it copious sinuses which embrace the intestine and supply its walls with blood. In Hirudo there are only a pair of lateral vessels, and neither dorsal nor ventral vessels; in this leech and in the Gnathobdellae generally there are intra-epidermic capillaries, a fact first discovered by Professor Lankester, and now known to occur also in the Oligochaeta.
The development of the blood-vessels shows that they have no relation whatever to the coelom, in spite of their subsequent connexion with it. The two longitudinal stems of Hirudo arise as cavities in the somatic layer of the mesoblast after the formation of the coelom. In Nephelis, but not in Hirudo, Dr. Bürger thinks that there is some reason for regarding the vascular system as the remains of the primitive segmentation-cavity of the embryo, an opinion which is held in respect of the vascular system of many other animals.
Fig. 205.—Glossiphonia marginata, vascular and alimentary system. ×4. (After Oka.) a, Dorsal vessel; g, intestinal caecum; v, one of the hearts.
Body-Cavity.—One of the most marked differences between the leeches and the Oligochaeta is in the body-cavity. In the latter there are a series of cavities corresponding to the segments, which are bounded in front and behind by the intersegmental septa, and in which all the viscera lie. In leeches such an arrangement is not recognisable save in Acanthobdella, where Kowalevsky[[455]] has quite recently described a typical coelom divided by septa into twenty segments. In transverse sections the body of other leeches appears at first sight to be solid, owing to the growth of the muscles and connective tissue. A more careful study, however, has revealed the fact that there are considerable remains of the body-cavity or coelom which form a complicated system of spaces and channels. What has happened, in fact, in the leech is that the coelom has become gradually and partially obliterated by proliferation of the cells in the interior of the body, a process of obliteration which has already commenced in the Oligochaeta. In many of the latter, some of the principal blood-vessels have become surrounded by a space cut off from the general body-cavity, while in the majority a special cavity surrounds the testes and the funnels of the sperm-ducts. This process of the formation of separate cavities for the inclusion of the several viscera culminates in the leeches with the marked obliteration of the greater part of the coelom. This has become so much reduced to the condition of narrow tubes that there has been a tendency to confuse it with the vascular system, more especially perhaps in those forms in which the blood is tinged with haemoglobin, and in which there is a connexion between the two systems of spaces. This confusion has been further increased by the plan of injecting the vascular system, a method of investigation which must be employed with great care in delicately-organised creatures whose tissues can be easily ruptured, and so lead to a flow of the injecting fluid into places and in directions impossible during life.
