FlG. 137. Hirudo medicitialis, the 

 medicinal leech ; general anatomy, 

 dorsal aspect. On the left side 

 onlythe alimentary canal is shown; 

 on the right the greater portion 

 of this has been removed to show 

 other organs. 1, head, with eve- 

 spots ; 2, muscular pharynx ; 3, first 

 diverticulum of the crop; 4. elev- 

 enth diverticulum of the crop; 

 5, stomach; 6, rectum; 7, anus; 

 8, cerebral ganglia; 9, ventral 

 nerve-cord; 10, nephridium; 

 II, lateral blood vessel' 12, testis; 

 13, vas deferens; 14, prostate; 

 15, penis ; 16, ovary ; 17, uterus. 

 (After Shipley and MacBride.) 



SYSTEMATIC ZOOLOGY 



this larva even becomes segmented, 

 but the segments disappear in the 

 course of the metamorphosis into 

 the adult worm. The Gephyrea are 

 therefore classified with the Annelida 

 chiefly because of this strong resem- 

 blance of some of their larvae to the 

 undoubted Annelida such as the Poly- 

 chieta. 



CLASS III. HIRUDINEA 



The Hirudinea(Lat. hirudo, leech ), 

 or leeches, live for the most part in 

 fresh water, though some are marine, 

 some live on land, and still others in 

 moist earth. They vary greatly in 

 size, and while the majority are from 

 five to ten centimeters in length, one 

 species is said to attain a length of 

 seventy-six centimeters. The land 

 leeches are common in India, South 

 America, and intermediate countries, 

 and, though only five centimeters long, 

 cause great annoyance to men and 

 horses by their large numbers and 

 persistent attacks. The water leeches 

 suck the blood from all animals living 

 in the water. 



In external appearance the Hiru 

 dinea differ from the other Annelida 

 in several points. The bodv is more 

 or less flattened dorsoventrallv and is 

 marked by a large number of rings. 

 which, however, do not all correspond 

 to true segments or somites, for there 

 are generally four or five of these 

 external rings to one internal segment. 

 Each animal is provided with two 



