PHYLUM ANNULATA 



517 



Systematic Position of the Example. 



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

 Gnaiholdcllida. 



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

 the possession of ten 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 Hirudinidffi : the genus 

 Hirudo is distinguished by the constant presence of 26 segments 

 and of 102 annuli. 



2.Clebsine 



3. Bra n c h e I li o n 



I.Ponl-obdella 



Fin. 413.— Three Rbyncbobdellida. Iir. 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 Macroldclla vcddivania is said 

 to attain a length of 76 cm. (2| 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 



