AXARTHROPODA. 131 



eyes whidi are placed on the head. The sexes are united in the 

 same individual. 



The most familiar of the Leeches are the common Horse-leech 

 (Hcemopsis), and the Medicinal Leech (Sanffuisuga officinalis). The 

 latter is provided with three semicircular toothed jaws (fig. 82, b, c), 

 which meet in a point, and are sufficiently powerful to cut through 

 the human skin. The Medicinal Leech is a native of fresh waters 

 throughout the south and east of Europe, and it is imported in large 

 numbers from Hungary, Bohemia, and Russia. 



In the Horse-leech the jaws are comparatively blunt, and the 

 animal has only the power of cutting through the soft mucous 

 membranes. It derives its name from the fact that it attaches 

 itself to the mucous membrane of the mouth in horses whilst drink- 

 ing at ponds or marshes. 



Order II. Oliqoch^ta. 



In this order are included the Earth-worms {Lumhricidce), and 

 the Water-worms {Naididce). They are all distinguished from the 

 preceding by the fact that the body is furnished with rows of 

 bristles (fig. 83, A), which take the place of the foot-tubercles of the 

 higher Annelida, and which are the organs of locomotion. They 

 are distinguished from the higher forms by the fact that the loco- 

 motive bristles are comparatively few in number, hence the modern 

 name of the order (Gr. oligos, few ; and chaiU, a bristle). In the 

 common Earth-worm {Lumhricus terresiris, fig. 83, B), the body is 

 cylindrical, attenuated at both ends, and furnished with eight rows 

 of short locomotive bristles. The mouth is destitute of teeth, and 

 opens into a gullet which leads to a muscular crop, succeeded by a 

 second muscular dilatation or gizzard. The intestine is continued 

 straight to the anus, and is constricted in its course by numerous 

 transverse partitions springing from the walls of the body-cavity, 

 and corresponding with the successive segments of the body. The 

 pseudohsemal system is well developed, and the dorsal vessel carries 

 a series of contractile dilatations or " hearts." The segmental organs 

 are long coiled tubes, which open by minute apertures on the 

 ventral surface, and communicate internally with the body-cavity. 

 The Ndididoe are chiefly noticeable on account of the power 

 which many of them possess of producing fresh individuals by a 

 process of budding before they attain sexual maturity. One of 

 the commonest of them is a little worm {Tubifex rimdorum, 

 fig. 83, A), which occurs abundantly in many of our pools and 

 streams, and which exhibits a fine red colour, owing to the 



