X] HIRUDINEA 1(31 



With the exception of Branchellion, which bears tufted gills, 

 the bodies of leeches are without external processes. There are 

 no parapodia, as in the Polychaeta, and no branchiae or tentacles, 

 and only one genus of the family has any chaetae. The body 

 .is segmented, and recently it has been shown that the number 

 of segments is always thirty-three. Some however of the segments 

 are fused together ; thus for example the posterior sucker contains 

 traces of six or seven true segments. The best test of the number 

 is to count the ganglia on the ventral nerve-cord. But even this is 

 not decisive, because although there are twenty-one free ganglia in 

 the centre of the body a certain number, some say five and some 

 six, are fused into the first ventral ganglion, and a certain 

 number, some say seven and some say six, coalesce to form the 

 ganglion of the posterior sucker. Whichever view is taken the total 

 number of segments is thirty-three. 



The body of the leech is ringed or divided into a number of 

 annuli. These do not, however, represent the segments, but a 

 number, varying in the different genera, make up a segment. In 

 ffirudo, the medicinal leech, there are five annuli to a true 

 segment ; in Clepsine, a common fresh- water leech, the number is 

 three. The real segmentation is, however, to some extent indicated 

 by markings on the skin. 



The animal is covered like the earthworm by a thin cuticle 

 secreted by the outermost cells, and the ectoderm contains numerous 

 goblet cells which are especially well-developed over the segments 

 abutting on the generative orifices. Here they form a clitellum, 

 and the secretion which the goblet cells pour out forms a co.coon 

 in which the eggs are laid. 



The nervous system of a leech does not differ in essentials from 

 that of the earthworm, but the nephridia, of which 

 Organ* 1 there are in Hirudo seventeen pairs, are peculiar. 

 They are no doubt a modified form of the same 

 organ as the nephridium of the earthworm, and they are best 

 described as U-shaped rods of cells. One end of the U com- 

 municates with the exterior by a muscular vesicle; the other 

 end is unconnected with anything else, but from the end con- 

 nected with the muscular vesicle a delicate cord of cells proceeds 

 inwards and terminates in a structure something like a minute 

 cabbage situated in a sinus above the testis. This termination is 

 called the testis lobe. The whole nephridium is traversed by a 



S. & M. 11 



