396 HIRUDINEA 



CHAP. 



original describer of the genus, gives of these chaetae, the part 

 implanted in the body is straight, while the part extending freely 

 beyond the body is sharply hooked. 



The body of the leeches is never ciliated externally ; there is, 

 as in the higher Oligochaeta, a cuticle secreted by the underlying 

 epidermis. The Hirudinea have, like the Oligochaeta, a clitellum 

 which, as in some of the lower members of that group, is limited 

 to a very few segments in the immediate neighbourhood of the 

 generative openings. It occupies in Hirudo segments 9, 10, 

 and 11. The epidermis gives rise to many unicellular glands 

 which either remain in situ or get moved to a deeper position. 

 In this the leeches exactly resemble the earthworms. There is 

 generally a great deal of connective tissue in the body-wall. The 

 muscles consist of circular, longitudinal, and radial series. The 

 individual fibres have the same structure as those of the Oligo- 

 chaeta, consisting of a soft and undifferentiated core, round which 

 is a radially-striated sheath of contractile substance. 



Alimentary Canal. The leeches are divided into the Ehynchob- 

 dellae, which have a proboscis but no jaws, and the Gnathobdellae, 

 which possess a series of jaws but have no proboscis. But the 

 division is not a hard and fast one, for we have Whitman's 

 genus Leptostoma, which should belong to the jawed division, but 

 which has quite rudimentary jaws without the sharp denticles so 

 characteristic of Hirudo. The pharynx is furnished with salivary 

 glands. The oesophagus is followed by the proventriculus, which 

 has a varying number of pairs of caeca ; then comes the intestine 

 and the rectum. The anus is, as a general rule, placed dorsally 

 to the sucker, but there are a few rare exceptions where the anus 

 is within the sucker. The caeca are totally absent in Trocheta. 



Vascular System. As will be seen from Fig. 205, the vas- 

 cular system of the Hirudinea is constructed on a plan which 

 closely resembles that of the Oligochaeta. The diagram re- 

 presents Glossiphonia, one of the Ehynchobdellae, the group 

 which comes nearer to the Oligochaeta in many particulars than 

 the Gnathobdellae. We can recognise a dorsal and a ventral 

 vessel, which are united in the anterior part of the body by 

 three perioesophageal rings, the elongation of which, particularly 

 of the last pair (v), from before backwards is very marked. In the 

 region of the sucker the dorsal and ventral vessels are united by 

 fourteen shorter loops, the number of which has an interesting rela- 



