Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/422

404 LEECH

Classification.

The group (Hirudinei or Discophori) may be divided into three families, viz., Rhynchobdellidæ, Gnathobdellidæ, and Branchiobdellidæ.

The Rhynchobdellidæ are those leeches furnished with a protrusible proboscis (which is often exserted if the animal is removed from the water and placed on a dry surface). This family includes the fish-leeches (Ichythyobdellidæ), which have an anterior and posterior sucker, a simple intestine, and mostly two pairs of eyes. Amongst these are Piscicola geometra, L., found on freshwater fishes, P. hippoglossi on the holibut, and P. respirans, in which the body has lateral sacs into which the blood enters. The first-mentioned (P. geometra) is a somewhat beautiful species, and full of activity, waving its body to and fro, and floating by aid of the expanded posterior sucker on the surface of the water. Another well-known genus (Pontobdella) is characterized by its thick warty skin, and four rings to each segment. The best-known example is the skate-leech (P. muricata, L.), which is olive-coloured and dusted with whitish grains. The anterior sucker is furnished with papillæ round its edge. It adheres to the skin of the skate, and deposits the curious pedicled horny capsules, containing a single egg, inside shells. In the same group is the remarkable genus Branchellion, which has a narrow nuchal region with the sexual orifice at its posterior part, and a series of frilled lateral appendages, the function of which has been supposed to be branchial. Its stomach is sacculated. One species (B. torpedinis, Sav.) is a messmate of the torpedo or electric ray of the Mediterranean; this has been the subject of very interesting papers by Leydig and De Quatrefages. The next subfamily – the Clepsinidæ –have somewhat broad bodies capable of being curved downward at the margins so as to form a hollow ventral groove for the lodgment of the eggs and the young, while the snout is pointed. They have from one to four pairs of eyes, and three rings to each segment. The dorsal blood vessel is rhythmically contractile, and the median blood-sinus envelops the digestive canal and the ventral nerve-cord. The stomach is branched, and the anus opens above the posterior sucker. The skin in many is warty, and in the Clepsine echinulata of Grube, from Lake Baikal, the dermal papillæ are furnished with soft pointed processes, so that in outline they are spinulose. The oviducts have no common tract or vagina, but open at the female pore. The genital apertures occur between the twenty-fifth and twenty-sixth, and between the twenty-seventh and twenty-eighth rings. The ova are in some kept under the body till hatched. Several species abound in the freshwater lakes and ponds of Britain, and their remarkable and beautiful anatomical structure is yet in need of elucidation. Amongst those most commonly met with is Clepsine bioculata, Sav. (fig. 2), which is about an inch in length, generally has a greenish-grey hue, and is much tapered anteriorly. Two closely approximated eyes occur in front. There is a reddish-brown body on the eleventh ring, marking an aperture described by O. F. Müller, and a whitish opacity in front of it. It often fixes itself by the posterior sucker, and waves its body to and fro in the water, and it swims actively like a Nemertean or horse-leech. The ova and young are carried in groups on the abdominal surface. It contracts itself into a ball on irritation. Its food consists of fluviatile and lacustrine mollusks, especially of Physæ (bubble-shells). Clepsine complanata, Sav. (fig. 3), again, is distinguished by its greyish-green or pale brown appearance, often with two (sometimes four) interrupted dark brown bands along the middle of the dorsum, in which are pale papillæ, four rows of the latter being generally present. The eyes are six in number, in parallel series. The body is firm, and the crenatures at the sides are never obliterated. There are six gastric sacs on each side; and in the young the rectum is ciliated. The proboscis is a cylindrical organ slightly narrowed anteriorly and posteriorly, and finely barred with transverse strife, a feature in C. bioculata due to the arrangement of the granular nucleated glands on its inner surface. It feeds on Planorbis and Limnæus (coil and mud-shells). Clepsine heteroclita, L. (fig. 4), a somewhat smaller form, is characterized by its translucent yellowish aspect. The dorsum is rather regularly dotted with pale brownish, so as to give it a checkered appearance. The snout is acute, and is furnished with six eyes, the anterior pair being closely approximated, while the two succeeding are separated by an interval from the foregoing, and the eyes in each pair are at a greater distance from each other. The digestive cæca are beautiful objects from their regularity and complexity. The ova are carried on the under surface of the body. It is less active than C. complanata. Another form very abundant under flat stones in similar lakes and ponds in certain places is Clepsine tessulata, O. F. Müller (fig. 5), a large and conspicuously tinted form. It reaches the length of 3 inches, and is of various shades of green, brownish, or olive, with six rows of yellowish or whitish specks, the marginal in all cases being the largest, while the four internal occupy papillæ. The eyes are eight in number in two series approximated in front. The soft, mobile, and almost gelatinous body is capable of assuming endless shapes, and is sometimes like a cordate leaf. It is gregarious in confinement. When a specimen is detached from its own adherent mass of ova, it occasionally selects another group and fixes itself to the glass to nurse them. The young are borne on the ventral surface (fig. 6). The genus Hæmentaria has two eyes, a bifid anterior sucker with the mouth in front, a long pointed proboscis, and five rings in each segment. The species (e.g., H. mexicana and H. officinalis) occur in the Mexican lakes and South America, the latter being used medicinally, since it is capable of penetrating the skin with its pointed proboscis.

Fig. 2. – Clepsine bioculata, Sav., and young. Dorsal view. Enlarged. Fig. 3. – Clepsine complanata, Sav. Dorsal view. Enlarged. Fig. 4. – Clepsine heteroclita, L. Dorsal view. Enlarged. Fig. 5. – Clepsine tessulata, O. F. Müller. Dorsal view. Somewhat enlarged. Fig. 6. – Clepsine tessulata, O. F. Müller. With a swarm of young adhering to the ventral surface. Slightly enlarged, as adhering to a glass vessel.

The second family, Gnathobdellidæ, includes the medicinal leech, besides Hirudo interrupta (M. Tand.) from Algiers, H. javanica from Java, H. sinica (Blainv.) from China, H. quinquestriata (Schm.) from Sidney, and others to be subsequently mentioned. H. decora (Say), the native leech of North America, is used in the same way as H. medicinalis. It is bluish, with about twenty-two reddish points on the dorsum and a lateral series of black touches of the same number. The ventral surface is ruddy with black points. It also comprises the genera Bdella, without denticles, and Hæmopis, the best-known example of which is H. vorax, M. Tand., a kind of horse-leech which is very troublesome to horses, cattle, and camels, by entering their nostrils when drinking; and the same disagreeable accident occurred to the French soldiers in Egypt. The common horse leech (Aulastomum gulo, Moq. Tand.), with very slightly developed