(574). On contemplating this singular dental apparatus found in the medicinal Leech, and considering the nature of the food upon which it usually lives, it is difficult to avoid arriving at the conclusion that such a structure is rather a provision intended to render these creatures subservient to the alleviation of human suffering than necessary to supply the wants of the animals themselves. In the streams and ponds where they usually inhabit, any opportunity of meeting with a supply of the blood of warm-blooded vertebrata must be of rare occurrence; so that comparatively few are ever enabled to indulge the instinct that prompts them to gorge themselves so voraciously when allowed to obtain it: neither does it appear that the blood which they swallow with so much avidity is a material properly suited to afford them nourishment; for although it is certainly true that it will remain for a considerable time in its stomach without becoming putrid, yet it is well known that most frequently the death of the Leech is caused by such inordinate repletion, provided the greater portion of what is taken into the body is not speedily regurgitated through the mouth.

(575). The internal digestive apparatus is evidently adapted, from the construction of all its parts, to form a capacious reservoir for the reception of fluids taken in by suction: the stomach, indeed, with the numerous lateral appendages opening from it on each side, would seem to fill the whole body; and, being extremely dilatable, allows the animal to distend itself to a wonderful extent, so that it is not unusual to see a leech, when filled with blood, expanded to five or six times the dimensions natural to it in an empty state.

(576). The stomach itself (fig. 107, h, i) occupies about two-thirds of the visceral cavity; on opening it, as represented in the figure, it is seen to be divided by delicate septa into nine or ten compartments that communicate freely with each other. In each compartment we observe two lateral orifices leading into as many wide membranous pouches (k), which, although shrunk and flaccid when in an undistended condition, as they are seen in the figure, are easily filled with fluid introduced into the stomach, and are then swelled out into very capacious bags. Perhaps the simplest way of obtaining a correct idea of the relative sizes and general arrangement of these organs is to make a cast of their internal cavities when in a state of distention; this is readily effected by placing a dead leech in warm water until it is slightly heated: in this state, the pipe of a small injecting syringe can be introduced into the oesophagus, so as to fill the stomach and caeca with common wax injection; and if the body be immediately removed into a vessel of diluted muriatic acid, the soft parts will be speedily destroyed, leaving an exact model of the interior.

It will then be seen that the lateral caeca increase gradually in size as they approximate the posterior extremity of the body, until the last pair (d) become so large as nearly to fill up the space intervening between the end of the stomach and the anal boundary of the visceral cavity.

Fig. 106. Dental apparatus of the Leech. A, triradiate arrangement of the teeth or saws; B, a tooth magnified.

(577). The small size of the intestine (e) when compared with the capacious stomach described above is remarkable: it commences by a minute orifice at the termination of the digestive cavity, and becoming slightly enlarged, passes in a straight line, lodged between the two posterior caeca, to the anus, which is an almost imperceptible aperture placed at the root of the posterior sucker: four small and apparently glandular masses are appended to this short canal; but their nature is unknown. The entire alimentary apparatus is retained in situ by numerous membranous septa (m m), passing between its outer walls and the muscular parietes of the body.

(578). In the Leech, the circulating system is more highly developed than in any other Annelid*. The presence or absence of a heart-like centre to this system in this class of animals is by no means the true criterion of the degree of its evolution. The amount of blood relatively to the size of the body, the degree of capillary subdivision which occurs on the periphery of the blood-system, and the proportion of the latter to the peritoneal fluid, form far more correct indications. In the Leech there exists no free space between the intestine and the integument: to this anatomical fact the highest interest will be shown to belong when explaining the mechanism of respiration in this Annelid. Here the chylous fluid, which in nearly all other Annelids occupies the general cavity of the body, like a cylindrical fluid stratum, separating the intestine from the integument, is transferred into the interior of the lateral diverticula of the stomach. The peritoneal chamber, being no longer required, is obliterated by the adhesion of the intestine to the integument: the union of these parts is effected through the medium of a dense spongy layer of capillary blood-vessels, the contents of which are exposed internally to the influence of the fluid contained in the digestive caeca, and externally to that of the surrounding element: hence the mechanism of the respiratory process, and the power enjoyed by this and other abranchiate Annelids of dispensing with all external breathing appendages.

Fig. 107. Digestive organs of the Leech (Hirudo medicinalis): b, mouth; h, i, interior of the stomachal cavity, exhibiting the openings of the lateral caeca (k); g, first pair of stomachal caeca; d, last pair, extending backwards on each side of the intestine e.