ARTICULATA. 97 



A species of leech finds its way under the eyelids, and into the nasal 

 passages ot Ardea virescens, in Martinique. In Algeria, JIcBmopis vorax is 

 abundant, and attacks man and animals upon land ; and it is found 

 sticking to tlie inside of the mouth and throat of slaughtered cattle in that 

 country. According to Baron Larrey, the French soldiers were much 

 j)lagued by a small species which attached itself to the entrance of the 

 throat. The same species was found in the nasal passages of the horse. 



As it is diiScult for a leech to affix itself to an animal, except at rare 

 intervals, it has been provided with peculiar digestive oi'gans to enable it to 

 undergo a long fast. The stomach is formed of a number of cells, each of 

 which has a lateral sac upon each side. The entrance from the stomach 

 to the intestine has a valve, and at the posterior extremity of the intestine 

 there is a sphincter to close it, so that the food may be admitted and 

 retained in small portions as it is required, and it is a remarkable fact, that 

 the blood thus kept in store (which may amount to nearly half an ounce) 

 retains its natural properties for many months. Hence the difficulty of 

 employing leeches in medicine when they have been gorged within some 

 months. This difficulty is obviated in some degree, by forcing them to- 

 disgorge a part of the blood by pressure, and a mode has been discovered 

 by piercing them with a lancet one third the length from the head, and 

 placing them in tepid Avater, when the contents of the stomach will escape, 

 especially if assisted by pressure. The wound heals in a few days, and the 

 leech may be applied again in fifteen or eighteen days. 



These animals are monoicous and oviparous, the eggs being deposited in 

 a kind of cocoon, resembling in size and form that of the silkworm, its 

 material resembling fine sponge. A Swedish species deposits its eggs in 

 the earth at soine distance from the water. 



Among the genera are Hirudo {H. officinalis^ ])l. 77, fig. 26), Malaco- 

 hdella {M. grossa, fig. 22), referred to in page 43 ; Clejmna {fig. 24), 

 Hcemopis {H. vorax^ fig. 25). Two species of leech are almost exclusively 

 used in medicine : Hirudo officmcdis, the green or Hungarian leech, the 

 belly of which is without spots ; and IT. inedicinalis, the German or brown 

 leech, with a spotted belly. 



The mouth of those leeches which are capable of biting, is tri-radiated, 

 each ray having a tooth which resembles a segment of a minute circular 

 saw, the margin of which is armed with a double row of microscopic 

 denticulations. The suction, whilst it draws the skin within the reach of 

 the teeth, renders it tense, so that they act like lancets, and the apparatus 

 answers the double purpose of a small scarificator and cupping-glass. An 

 examination of the wound made by a leech will show the tri-radiated 

 arrangement of the teeth. Artificial leeches are sometimes made use of, 

 the quality of which must greatly depend upon the accuracy with which 

 the natural instrument is imitated. 



Three species of Hirudo, and one of Clepsina, have been described from 

 the waters of the United States. 



It was at one time believed that changes in the weather might be 

 foretold from the actions of leeches, but the value of these indications wa& 



301 



