X 



PHYLUM ANNULATA 



517 



Systematic Position of the Example. 



Hirudo belongs to the family Hirudinidce, of the sub-order 

 Gnathobdcllida. 



The presence of jaws places it in the sub-order Gnathobdellida : 

 the possession often eyes, and the presence of five rings to all the 

 segments except a few at the anterior and posterior ends, dis- 

 tinguishes it as a member of the family Hirudinida^ : the genus 

 Hirudo is distinguished by the constant presence of 26 segments 

 and of 102 annuli. 



2. Clefasine 



S.Bronc hellion 



I.Pon ^obdella 



Fin. 413. Three Rhynchobdellida. br. gills ; pr. everted proboscis. (1, after Bourne ; 2 and 3 



after Cuvier.) 



3. GENERAL ORGANISATION. 



In the essential features of their organisation the Leeches are a 

 very uniform group : there are, however, a few interesting modifi- 

 cations of structure which must be referred to. 



Form and Size. Most kinds do not exceed a few centimetres 



in length, but the American species Macrdbdella valdivania is said 



to attain a length of 76 cm. (2J feet). The number of annuli to 



a segment varies from three to five, but the general form of the 



body is remarkably uniform, the external differences between 



