236 INVERTEBRATE MORPHOLOGY. 



consist of a number of pairs, varying from twelve or more to 

 six (Clepsine}, of spherical bodies lying in the same region of 

 the body as the ovaries. Each testis has its own duct, which 

 opens into a longitudinal vas deferens common to all the 

 testes of the same side of the body. Anteriorly the two vasa 

 deferentia unite to open in the mid-ventral line of usually the 

 tenth metamere (Tig. 106, mo), frequently through a strong 

 muscular penis (Fig. 108, pe). In many forms special glan- 

 dular thickenings, supposed to be useful in copulation, occur 

 'on the ventral surface of one of the metameres behind that 

 bearing the opening of the oviduct (Fig. 106, gc). 



The Hirudinea are at present usually divided into two 

 orders, though it seems probable that further division of one 

 of them will be necessary later. 



1. Order Gnathobdellidse. 



In this order are included the leeches which are provided 

 with chitinous jaws in the walls of the muscular pharynx. 

 In addition to this all the members of the order are charac- 

 terized by possessing five rings to each fully developed meta- 

 mere. To this order belong the Hirudinidse, characterized by 

 possessing ten eyes arranged in pairs on the five anterior 

 metameres behind the prostomium, and including Hintdo, the 

 medicinal leech, a native of Europe, instead of which Macro- 

 Mella is sometimes used in America. The Nephelidge, with 

 the genus Nephelis, differ in possessing fewer eyes (four pairs), 

 and in having distinct segmental sense-organs either wanting 

 or occurring on all the rings of each segment. 



2. Order Rhynchobdellidse. 



The Bhynchobdellidse are characterized by possessing a 

 protrusible pharynx, as well as by possessing three, six, or 

 twelve rings to a metamere. In the Ichthyobdellidse, or fish- 

 leeches, the larger numbers are found, the number six being 

 characteristic of Pontobdetta, while twelve occurs in Piscicola. 

 In the Clepsiuidie but three rings are found to each meta- 

 mere, and the eyes are either two or six in number. To this 

 family belongs the genus Clepsine, a common fresh-water 

 form, as well as the tropical laud-leech, Hcementeria. 



