HIRUDINEA 267 



The Hirudinea may be divided as follows : 



AcANTHOBDELLiDAE, a family intermediate between the Oligochaeta 

 and the Hirudinea, containing the single genus Acanthobdella. 



Rhynchobdellidae, marine and freshwater forms, with colourless 

 blood, protrusible proboscis and without jaws. 



Gnathobdellidae, freshwater and terrestrial forms, with red 

 blood, without a protrusible proboscis but usually with jaws. 



Family Acanthobdellidae. 



Acanthobdella (Fig. 196 A), a parasite of salmon, is a link with the 

 Oligochaeta. In it the specialized hirudinean characters are only partly 

 developed. There is no anterior sucker but a well-developed posterior 

 sucker formed from four segments. The total number of segments is 

 twenty-nine compared with thirty-two in the rest of the group. There 

 are dorsal and ventral pairs of chaetae in the first five body segments 

 and the coelomic body cavity is a continuous perivisceral space, in- 

 terrupted only by segmental septa as in the Oligochaeta. It is, however, 

 restricted by the growth of mesenchyme in the body wall and split 

 up into a dorsal and ventral part in the clitellar region. The so-called 

 testes (really vesiculae seminales) are tubes running through several 

 segments, filled with developing spermatozoa and their epithelial wall 

 is continuous with that of the perivisceral coelom, another primitive 

 feature. The vasa deferentia, moreover, open into the testes by typical 

 sperm funnels. 



It is interesting to find that in the Branchiobdellidae, a family of 

 the Oligochaeta, parasitic on crayfish, there is the same sort of leech- 

 like structure: a posterior sucker, annulated segments, absence of 

 chaetae and presence of jaws. But the condition of the coelom, 

 nephridia and generative organs is so like that of the OHgochaeta that 

 the family must remain in that group. 



Family Rhynchobdellidae. 



Pontohdella^ parasitic on elasmobranch fishes. 



Glossiphonia (Fig, 195), a freshwater leech feeding on molluscs like 

 Limnaea and Planorhis and on the larvae of Chironomus ; body ovate 

 and flattened ; hind gut with four pairs of lateral coeca ; eggs laid in 

 the spring, the young when hatched attaching themselves to the 

 ventral surface of the body of the mother. 



Family Gnathobdellidae. 



Hirudo^ the medicinal leech, at one time a common British species 

 but now extinct; jaws armed with sharp teeth. 



Haemopis^ the horseleech, common in streams and ponds, which it 

 leaves to deposit its cocoons and in pursuit of prey ; jaws armed with 



