132 
The British Leeches 
selves into a series of regularly recurring groups corresponding to the 
successive somites of the body. 
Towards the ends of the body the number of rings in a group 
becomes smaller, and we frequently find at the extremities one or 
more somites represented respectively by only one ring. 
For the sake of brevity the ring or group of rings corresponding 
to the metameric divisions of the body are themselves alluded to as 
somites (segments, zoonites), and for each of the similar groups con¬ 
taining an equal number of rings, which occur throughout the greater 
part of the body of every leech, the term complete somite has been 
retained. 
In the number of rings of which a complete somite is composed, 
in the number of such complete somites, in short, in an analysis of the 
external metamerism, we have characters of the greatest importance in 
the determination of genera and species. 
In the following descriptions I have not adopted the neuromeric 
standard of somite limits advocated by Moore (1900) and Castle 
(1900 b) and since supported by Livanow but, as far as the delimitation 
of somites is concerned, have adhered to the original conception of 
Gratiolet (1862) which has subsequently been elaborated and supported 
by writers of such divergent opinions as Whitman and Blanchard on 
the one hand and Apathy on the other. The first riug of the complete 
somite, as here understood, is the sensory ring which lodges a ganglion 
of the ventral chain and bears externally the “ metameric sensillae ” 
(Whitman) often rendered conspicuous by association with special 
colour markings and by elevation upon more or less prominent 
papillae. It has long been recognised that a somite is determined 
by the presence of a ganglion and that in the central nervous system 
we have twenty-one free single ganglia with a mass of fused ganglia 
at either extremity. Apathy (1888 b) found six ganglia in the circum- 
pharyngeal ganglionic mass, whilst in the posterior ganglionic mass 
Whitman (1892) found seven; and these results have been confirmed 
by subsequent workers. 
Thus there are 34 ganglia and somites in the body of the leech. Of 
these somites, seven are absorbed by the posterior sucker and con¬ 
sequently we have to account for 27 in the rest of the body. The first 
four somites and sometimes part of the fifth may be involved in the 
formation of the anterior sucker, and the genital apertures, as Apathy 
first pointed out, invariably occur, the male in somite XI and the 
female in somite XII. This rule holds good even in the case of 
