Fig. 201.—Anterior end of Macrobdella sestertia, to show eyes and sense bodies. (After Whitman.)
The use of Hirudo medicinalis is well known to many of us from personal experience. So extensively was this leech formerly made use of that it is now far from being a common species either in this country or in France. Those who desire full information as to Hirudiniculture should consult the work of Dr. Ebrard, published in 1857.[[452]] The former extensive use of the leech has led to the transfer of its name to the doctor who employs it, the authors of the sixteenth century constantly terming a physician a leech; it has been suggested, however, that the term was applied rather by way of analogy. The useful blood-sucking habits of the medicinal leech have been wrongly attributed to the innocent horseleech (Aulastomum)—innocent, that is to say, of the blood of Vertebrates, for it has been described as "a cruel and greedy worm," engulfing earthworms and even smaller specimens of its own species. "Horsleches," said an old writer, "are wholesome to drawe foorthe foule blood, if thei are put into a hollowe rede, and one of their endes cutte of, whereby the blood maie run forthe." But it is clearly not easy for a creature destitute of jaws and teeth to bite, and the similarity of general aspect has doubtless led to a confusion with the savagely biting medicinal leech.
Fig. 202.—Sense body of Macrobdella sestertia. (After Whitman.) ep, Epidermis; s, clear cells. Highly magnified.
Fig. 203.—Section through eye of Haemadipsa japonica. (After Whitman.) ep, Epidermis; n, nerve. Highly magnified.
The Hirudinea are all distinctly segmented animals, but the segmentation differs from that of the Oligochaeta in two points. In the first place the number of segments is much smaller in a leech than in an Oligochaete, although the difference does not appear great at first sight.
A leech's body may seem to be composed of seventy, eighty, or one hundred segments, a number quite as great as is found, for example, in the genus Perichaeta among the earthworms; but the apparent number of segments in the leech is produced by a very marked annulation of the real segments; and this is indeed the second point of difference referred to above. But there are earthworms which show frequently a secondary annulation,—secondary because it appears late and does not affect other organs of the body. A segment of an earthworm may indeed have five or six distinct annulations, but it will be bounded internally by two septa, and will bear only one set of chaetae externally. In the leech external clues to the definition of a segment were until recently wanting. They appear now to have been found in the sensory organs of the skin (Figs. 201 and 202), which are, according to Whitman,[[453]] disposed in a perfectly metameric fashion. Judged by this, and also by the nephridia and nerve-ganglia, the number of segments in a leech does not appear to exceed twenty-six, independently of the sucker, which may represent a few fused segments, seven (in the medicinal leech) according to Leuckart.
The eyes, which are so useful in the systematic arrangement of the group, appear to have been evolved from these sensory organs by a further exaggeration of their peculiarities. Figs. 202 and 203 show this point convincingly. The segmental sense organ is shown in Fig. 202; to the outside of certain sense cells, below which are a mass of ganglion cells, are certain peculiar transparent cells very similar to the clear cells found in the interior of the eye (Fig. 203). The segmental disposal of the sensory bodies and of the eyes is shown in Fig. 201.
