ANNELIDA. 75 



the chromatin elements of the pronuclei become intimately united, and 

 that when the oosperm undergoes cleavage, the chromatin is halved at each 

 cell- division in a very exact manner. Centrosomata in the protoplasm play 

 an important part in cleavage. 



9. HIRUDO (The Leech). 



The medicinal Leech (Hirudo medicmalis) is a slimy, flattened 

 worm-like animal some 2 to 6 inches long, found abundantly in 

 the freshwater pools and marshes of the Continent. It also 

 occurs in this country. There is a sucker at each end of the 

 body, and by the alternate attachment of these locomotion is 

 usually effected. The animal can also swim by means of wave- 

 like contractions of the body. It is a matter of ordinary know- 

 ledge that the Leech is a blood-sucking ectoparasite. 



MORPHOLOGY AND PHYSIOLOGY. 



1. External Characters. The bilaterally symmetrical body, 

 when fully extended, is somewhat strap-shaped, broadest about 

 the middle, and with the dorsal surface rather more convex than 

 the ventral. A large amount of contraction can, however, take 

 place, by which the body is much shortened and its shape altered. 

 A number of transverse grooves divide the body into about 

 95 rings or annuli, these, however, being superficial and not 

 corresponding to the true segments, which are 26 in number, 

 as indicated by the structure of the nervous and excretory 

 systems. Except near the ends each segment is made up of five 

 annuli, of which the last is distinguished by special colour- 

 markings, and the first by a transverse series of small white 

 papillae. There is a considerable difference in appearance between 

 the dorsal and ventral surfaces, for the former is much darker 

 and marked on each side by three longitudinal bands, of which 

 the two outer are diversified by dark dots, specially conspicuous 

 in the last annulus of each segment. The segments making up 

 the anterior and posterior ends are shorter than the others and 

 contain fewer annuli. 



An oval ventral sucker (Fig. 23) terminates the front end, and 

 it appears to be formed by fusion of a lip-like prostomium with 

 the first two annuli of the most anterior or peristomial segment. 

 The mouth is placed within this sucker, and round its dorsal 



