PHYLUM ANNULATA 



477 



mth 



is absent in some land-leeches, and in other species all three jaws 

 are rudimentary or absent. 



The varying development of the blood-vessels and sinuses 

 presents many points of interest tending to explain the condition 

 of things described above in the medicinal leech. In the latter, 

 as we have seen, there are lateral vessels with contractile muscular 

 walls, and dorsal and ventral sinuses with non-contractile walls. 

 In Pontobddla, one of the Rhynchobdellida, there are dorsal 

 (Fig. 381, 2, d.v.} and ventral (v.v.) as well as lateral vessels, and 

 lateral (7.s.) as well as dorsal and ventral sinuses, and in each case 

 the vessel is enclosed in the corresponding sinus. The ventral 

 sinus (v.s.~) also con- 

 tains the nerve-cord 

 (n.c.') and the ovaries 

 (ov.), and offshoots 

 of it surround the 

 testes (ts.~) and the 

 nephrostomes (nst.). 

 This arrangement 

 clearly suggests the 

 partial obliteration, 

 by growths of con- 

 nective tissue, of an 

 originally continu- 

 ous ccelome. Another 

 interesting condi- 

 tion occurs in Ne- 

 phelis (3\ in which 

 the middle region 

 of the body contains 

 a series of paired, 

 metamerically ar- 

 ranged spaces (c.), 

 surrounded by bo- 



tryoidal tissue, and .containing the nephrostomes. Development 

 seems to show that these cavities are derived from true ccelomic 

 spaces in the embryo, formed, as in Chsetopoda, by a splitting of 

 the mesoderm in each segment. 



In most instances the skin, with its abundant supply of capil- 

 laries, constitutes the only respiratory organ, but in Branchellion 

 (Fig. 379, 3) a Rhynchobdellid parasitic on the Electric Rays 

 (Torpedo and Hypnos) and on one of the Australasian Skates (Raja 

 nasuta), differentiated respiratory organs or gills (br.) are present 

 in the form of delicate lateral outgrowths of the segments. 



In most members of the class the nephridia are formed on 

 the same general type as those of Hirudo, but differ in the 

 structure of the nephrostomes, which may be ordinary ciliated 



gui 



cr 



C 1 



cr 



FIG. 380. Proboscis of Clepsine. A, retracted ; B, everted ; 

 cr. crop; gill, gullet; iiitlt. mouth; pr, introvert; s. ffl. 

 salivary glands. (After Bourne.) 



