NEREIS CULTRIFERA 179 



then lengthening and segmenting. Their mesoderm is formed 

 as two ventro-lateral bands, each thrown off by the continual 

 division of a pole cell at the hinder end, and spreads round the 

 gut, between ectoderm and endoderm, the coelom appearing in 

 it. The larva of Nereis is not in all respects a typical trochosphere. 

 In Fig. 127, B a more typical example is shown, in a later stage 

 of development than that represented in Fig. 127, A. It has 

 between ectoderm and endoderm a large space (blastocoele) 

 which is not present in the trochosphere of Nereis. 



HIRUDO MEDICINALIS 



THE MEDICINAL LEECH 



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, was 

 becoming rare, as readers of Wordsworth will remember, a 

 century and a half ago, but it is still found in the Lake District, 

 in the New Forest, and in some other places in Great Britain. It 

 is commoner 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 when it is full grown it also takes that of warm-blooded 

 animals, and it 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 three or four cubic 

 centimetres of blood. The body of the leech varies in length, 

 according to the state of contraction ; at its maximum it is about 

 six inches and somew^hat flattened, and it is provided at each 

 end with a downward-facing sucker. It is encircled by 95 minute 

 rings or annuh (Fig. 128), and brightly marked in various shades 

 of green, yellow, and black, paler below than above. The annuh 

 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 (prostomium and first five segments) there are eleven 

 annuh, 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 32, 

 including those of the head and hinder sucker. The mouth lies 



