234 MANUAL OF ELEMENTARY ZOOLOGY 



consisting of temporary masses of cells, which arise from 

 the ccelomic epithelium. The ova and sperm probably 

 escape by temporary openings formed in the body-wall, 

 and fertilisation takes place in the water. The free 

 young are at first very unlike the parents, being minute, 

 globular creatures, known as trochosfiheres, which swim by 

 means of a girdle of cilia in front of the mouth and have 

 an apical tuft at the front end. They undergo a gradual 

 change into the adult, becoming oval and then lengthening 

 and segmenting. 



The leeches are another group of segmented worms 

 related to the earthworms. Hirudo medicinalis, 

 the Medicinal Leech, a dweller in freshwater 

 pools, marshes, and sluggish streams, is found sometimes 

 in this country but more commonly on the Continent, 

 where, when it was more used in medicine than at present, 

 it was bred in large numbers in special ponds. It lives 

 normally by sucking the blood of frogs and fishes, but also, 

 when it is full grown, on that of warm-blooded animals 

 which enter its haunts, and will feed on man, though to 

 induce it to do so his skin may have to be moistened with 

 blood or milk or pierced by a small cut. An active specimen 

 will draw one or two drams of blood. The body of the 

 leech is 3-5 inches in length, somewhat flattened, and pro- 

 vided at each end with a downward-facing sucker. It is 

 encircled by 95 minute rings or annuli, and brightly marked 

 in various shades of green, yellow, and black, paler below than 

 above. The annuli do not indicate the true segmentation. 

 In the greater part of the body five of these lesser rings go to 

 a segment, but towards the ends there are fewer, and in the 

 head or region of the anterior sucker there are eight annuli 

 to five segments, while the posterior sucker represents seven 

 segments fused without annulation. Unlike that of the 

 earthworm or Nereis, the number of segments is a definite 

 one, amounting in all to 33, including those of the head 

 and hinder sucker. The mouth lies in the midst of the 

 anterior sucker, and the anus is a minute opening above the 

 base of the hinder sucker. The male and female genital 

 openings are median on the fourth annuli of the 10th and 

 nth segments respectively. On the first annulus of the 

 seventh segment, and the second annulus of each from the 



