CHAPTER XX 

 THE LEECHES (HIRUDINEA) 



By J. PERCY MOORE 



Professor of Zoology in tht University of Pennsylvania 



The Hirudinea or leeches are predatory or paras'tic annelids 

 with terminal suckers serving for attachment and locomotion. 

 Quite nearly related to the Oligochaeta and closely resembled by 

 the semi-parasitic Discodrilidae in the possession of suckers, jaws, 

 and median genital orifices and in the absence of setae, they are 

 characteristically modified for procuring and digesting their pecul- 

 iar food, consisting typically of blood and other animal juices. 



The body of a leech is generally constituted of thirty-four meta- 

 meres (designated I to XXXIV), each represented in the central 

 nervous system by a ganglion usually consisting of six capsules or 

 groups of nerve cells. Externally superficial furrows divide each 

 fully developed somite into from two to sixteen rings or annuli. 

 One of these, lying at the middle of the somite, contains the ganglion 

 and usually bears three or four dorsal pairs and three ventral pairs 

 of eye-like sense organs or sensillae and is termed the neural or 

 sensory ring. Segments having the full number of annuli charac- 

 teristic of the genus are termed complete, and are always found in 

 the middle region. Incomplete or abbreviated segments occur at 

 the ends of the body and may have any number of annuli less 

 than the complete somites into which they grade. Recognizing 

 the triannulate somite as basic for most leeches and considering 

 that more complex somites may be derived by repeated binary 

 division of its annuli the following symbols are employed for the 

 precise designation of particular rings. Counting from the head 

 end the rings of the triannulate somite are A^, A^, and A^, where 

 A^ is the neural or sensory annulus. These, bisected, give col- 

 lectively the secondary annuli B^ to B^. Repeated subdivisions 

 give tertiary annuli C* to O^ and quaternary annuli D^ to D^^. 

 But the full theoretical number of the fourth order is nevet 



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