Prepared to show its laterally sacculated stomach and the intestine. A stiffening injection of gelatine was thrown into the digestive tube, the specimen hardened in spirit and then dissected.
The integument has been divided down the middle dorsal line and reflected to either side: the portions of the vascular system interposed between the digestive tract and the body-walls cut away, and the entire cavity of the 'crop' and its diverticula exposed by the removal of their dorsal wall. Anteriorly to the crop is the pharynx with a villous exterior. This appearance is due to the presence of numerous unicellular salivary glands and to the cut ends of radial muscular fibres. The pharynx is succeeded by the crop which has thin walls. Lateral diverticula or caeca which occupy five-sixths of the entire body are appended to it on either side. These caeca are really segmental dilatations of a central tube. There are eleven of them in all. The first is small. The second appears to be double. The third has much the same appearance but not so well marked. It is due to the great development of a partial septum which may be seen in the succeeding seven caeca projecting backwards from the anterior wall. In some species of Clepsine the caeca are very distinctly bifid at their outer ends.
The last pair of caeca, the only pair in Aulostoma, are of very great length and bending sharply almost immediately at their commencement so as to become apposed to each other along the middle line, are prolonged backwards to a point on a level with the commencement of the rectum, and nearly as far as the end of the body. The form of the caeca, it should be noted, depends very much on their state of distension. This region of the digestive tract serves, first, as a crop or reservoir for the blood which forms the sole food of the animal, and, secondly, as a place where slow changes go on in the various constituents of that fluid. The oxy-haemoglobin is extracted by degrees from the corpuscles: it is reduced to haemoglobin and crystallised, and changes gradually take place in the corpuscles.
Between the two last caeca lies the stomach or 'gastro-ileal' (Gratiolet) section of the alimentary canal. A black bristle has been passed into it. It communicates by a narrow aperture with the crop, and at its commencement there is a pair of small caeca, one on the right, the other on the left, directed forwards. These caeca and the stomach are much larger in the Horse-leech (Aulostoma gulo). The stomach is very vascular, and it has a villous interior with a spirally arranged valve. The contents of the crop enter into it very slowly, and the red colour of the blood then changes from dark-red to green. The posterior end, colon or intestine, of this section of the alimentary canal is little vascular and contains no valve. It is followed by a short rectum of small calibre which terminates in a dorsally-placed anus, as in all Leeches except Acanthobdella where it opens in the centre of the posterior sucker. From a developmental point of view the pharynx and rectum must be regarded as invaginations from the exterior, i. e. as stomodaeum and proctodaeum, while the rest of the canal is archenteron, i.e. lined by endoderm or hypoblast.
The ventral nerve chain may be seen in part through the walls of the stomach. The spot where the walls of the caeca meet centrally (i. e. in the median tube) corresponds very nearly in most instances with a ganglion.
On either side of the crop and adherent to the inner surface of the reflected body-walls may be seen remains of the botryoidal tissue. It is arranged chiefly in four bands, two dorsal, two ventral, close to the walls of the digestive tract but separated from it by a layer of vaso-fibrous tissue and capillaries.
A section taken through the pharyngeal region shows, according to Gibbs Bourne, the following structures passing from within outwards: (1) Pharyngeal epithelium composed of minute cells; (2) three ridges, one dorsal, two lateral, composed of salivary ducts and radiating muscles, the branched ends of which abut upon the hypodermis; (3) a circular layer of muscles very dense and compact; (4) the longitudinal muscle layer of the body wall, in which occur blood sinuses, vessels, and salivary glands; (5) the diagonal and circular muscle layers of the body; and (6) the hypodermis and cuticle. To the three pharyngeal ridges correspond the three muscular jaws. They bear at their edges in the medicinal Leech about 80-90 fine chitinoid teeth. These teeth contain lime carbonate both in Hirudo and Aulostoma. Haycraft (P. R. S. xxxvi. 1883-84, p. 478) has proved that the secretion of the pharyngeal glands has the power of arresting coagulation in blood apparently by destroying the coagulation ferment. The muscle cells of the 'crop' are for the most part disposed in a transverse direction, and their ends are branched. Its epithelium is low, columnar, and the cells, according to Gibbs Bourne, may be seen giving off clear droplets into the blood. In Aulostoma the alimentary epithelium is ciliated.
Its food consists of worms, etc. Moquin-Tandon states that when young this Leech possesses distinct lateral caeca to the crop in addition to the posterior pair. The region of the crop is not sacculated in Nephelis, Trocheta, or Pontobdella. In the last two it is constricted at intervals, and the last-named possesses a posterior azygos caecum underlying the stomach. Trocheta is carnivorous like Aulostoma.
The body-cavity or coelome is in all adult Leeches almost obliterated by connective tissue growths. This process of obliteration of the coelome is termed by Gibbs Bourne diacoelosis. The remains of the coelome are much more conspicuous in the Rhyncho- than in the Gnathobdellidae. In the medicinal Leech its chief remains are in the form of the dorsal and ventral sinus, the latter lodging the nerve-cord. They are in direct connection only by means of the dorsal sinus of the gastro-ileal section of the alimentary canal. Other remains of the coelome are found in the network of vessels surrounding the testes, etc. The blood-vessels, which with their branches have muscular walls, are represented by a right and left longitudinal trunk which anastomose at the anterior and posterior extremities of the body. These trunks give off in each somite latero-abdominal vessels which anastomose ventrally, and two sets of dorsal vessels, short latero-lateral and long latero-dorsal branches. In the gastro-ileal region the latter anastomose inter se dorsally by means of their posterior branches. Anteriorly to this region they are connected only through the capillary system of thin-walled vessels.
There is a superficial network of fine capillaries which penetrate the hypodermis, and more deeply pass into an intermediate layer connected with the lateral vessels and with the botryoidal tissue. The latter forms the deepest layer, connected on the one hand with the vessels given off from the lateral vessels, on the other hand with the sinuses. It tends to form a secondary coelome (= metacoelosis, Gibbs Bourne). The dorsal and ventral sinuses, according to Gibbs Bourne, communicate with (1) the cutaneous network, (2) the capillaries of the crop, and (3) of the stomach, and (4) the sinuses (moniliform hearts of Brandt) which surround the nephridial funnels and lie upon the testes.
The thin-walled capillaries of the Leech possess no endothelium and no nuclei. The walls of the vessels in the botryoidal tissue are formed solely by the pigmented cells themselves. The blood-plasma in the Gnathobdellidae, but not in Rhyncho-bdellidae, is coloured red by haemoglobin. It contains amoeboid corpuscles, and here and there Prof. Ray Lankester detected nuclei set free from the walls of the developing capillaries, as in the Earthworm. But they appear to be very scanty in number in the Leech.
Bite of Leech, Carlet, C. R. 96, 1883. Lime in teeth, Schneider Zool. Beitrage, i. 1885.
Alimentary canal. Gratiolet, A. Sc. N. (4), xvii. 1862, p. 182; Gibbs Bourne, op. cit. p. 492.
Digestion of blood by the common Leech. Stirling, Journal Anat. and Physiol. xvi. 1882.
Vascular system and coelome. Gibbs Bourne, op. cit. p. 453; Jaquet, Mittheil. Zool. Stat. Naples, vi. 3, 1885.