676 



VERMES— ANNELIDA. 



bearing species, and the lihynclwhdtlUdm, or proboscis-bearing species. Of the 

 Gnathobdellidie, the medicinal leech (Hirudo medicinalis) is a well-known 

 example. This animal is usually of a blackish colour, and ornamented with 

 longitudinal bands of yellow spots. It is somewhat scarce in England, 

 and in the days when blood-letting was extensively practised by doctois 

 the demand for it was met by the importation of large quantities 

 from the Continent, where it is not only more abundant than with 

 us, but was artificially cultivated on a large scale in especially-constructed 

 tanks. The power of these animals to pierce the skin and draw blood, upon 

 which their value as blood-letters depends, is due to the presence in the 

 mouth of three semi-circular jaws, each of which 

 is studded with a series of minute horny teeth. By 

 means of a set of powerful muscles these jaws are 

 capable of being worked backwards and forwards 

 after the manner of a saw, while by aid of the 

 sucker, the mouth is kept closely applied to the 

 wounded spot and the blood is drawn into the 

 worm's stomach. This is a capacious sac, consisting 

 of no fewer than eleven pairs of pouches, capable of 

 considerable distention, and occupying the greater 

 part of the space between the walls of the body. 

 When the leech has pumped itself full of blood, but 

 not before, ' it relaxes its hold ; and it has been 

 affirmed that the amount thus imbibed will be 

 sufficient for a whole year. 



In a natural state these leeches are found in 

 ponds, marshes, or sluggish brooks with a bottom 

 of mud or clay. Out of water they cannot live for 

 any length of time, since they speedily succumb to 

 the drying effects of the atmosphere. During the 

 day, especially if the weather be warm and bright, 

 they may be seen swimming about with liveliness, 

 but in cold or dark days and at night-time they lose their activity, and remain 

 curled up with head and tail closely applied. In the autumn they bury them- 

 salves deep in the mud. Their food consists of the blood of fishes or aquatic 

 amphibians, or of terrestrial vertebrates, that visit their haunts to bathe or 

 drink. Like earth-worms, leeches are hermaphrodite, and after pairing in the 

 spring they creep up the banks of the pond or ditch above the surface of the water 

 and dig burrows in the soft damp soil, and in these, like the earth-worms again, 

 they construct for the reception of their eggs elliptical cocor ns, formed of a 

 secretion from certain skin-glands, which afterwards hardens. Externally, 

 the cocoons are covered with a layer of spongy material, while the inside is 

 filled with a.bumen, in which the small eggs, usuallj about twenty in number, 

 are embedded. When the esgs are hatched, the young float in the albumen, 

 and obtain their nourishment from it. Upon quitting the cocoon after 

 several weeks' residence therein, the young leeches resemble their parents in 

 form, though they are not dark-coloured. Growth, however, is a slow pro- 

 cess, maturity not being reached until five years have passed, and life endur- 

 ing, under favourable conditions, for about twenty years. 



A far commoner species in England than the medicinal leech is the so-called 

 Horse-leech {Aidosto-imiin jfn'o), which may be distinguished from the former 

 by the absence of the bands of yellow spots, the akin being of a uniform 



Fig. 6. — Medicinal Leech 

 (Hirudo medicinalis). 



