300 THE INVERTEBRATA 



system communicates both with the coelom and the blood system. 

 The pecuhar functions of the lymphatic system are not shared by 

 the botryoidal vessels which have no particular connection with the 



The nervous system is of the usual annelidan type but characterized 

 by the fusion of ganglia anteriorly (Fig. 205) and posteriorly. There 

 are segmental sense organs in the form of papillae, and on the head 

 some of these are modified to form eyes and the so-called "cup- 

 shaped organs". 



The nephridia consist of two tubes, one ending in a nephrostome, 

 the other with an external aperture ; their lumina do not communicate 

 (Fig. 192); the nephrostomes open into a branch of the ventral or 

 the lateral sinus. The testes^ of which there are often several pairs 

 (nine in Hirudo), and the single pair of ovaries are also present as closed 

 vesicles in the sinuses and are derived from the coelomic epithelium, 

 but in distinction from the rest of the annelids they are continuous 

 with their ducts. The separation of the genital part of the coelom from 

 the rest, begun in the Oligochaeta, here becomes complete. The 

 testes discharge into a common vas deferens on each side; the two 

 vasa unite anteriorly to form a median penis. Similarly the two 

 oviducts join and the eggs pass through a single albumen gland and 

 vagina to the exterior. The spermatozoa, united in bundles, are 

 deposited on the body of another leech and appear to make their way 

 through the skin to the ovaries where fertilization occurs. The eggs 

 are laid in cocoons, the case of which is formed by clitellar glands in 

 the same way as in Lumbricus. 



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. 207 A), a parasite of salmon, is a hnk 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 



