HIRUDINEA 



299 



blood system consists of two contractile lateral vessels (and in the 

 Rhynchobdellidae of dorsal and ventral vessels running inside the 

 corresponding coelomic spaces). These vessels all communicate with 



m.c. 



ect. 



t. n.s. 



v.s 



n.c. 



Fig. 207. Transverse sections of Hirudinea to show the progressive restriction 

 of the coelom. A, Acanthobdella, B, Glosstphonia, C, Hirudo. In A the coelom 

 {coe.) is continuous but encroached upon by growth of parenchyma (stippled). 

 In B it is broken up into a system of sinuses, d.s. dorsal; v.s. ventral; h.s. 

 hyperdermal sinus ; l.s. lateral and i.s. a network of intermediate sinuses. In 

 C the sinuses (outlined in black) are reduced in size, and there is no inter- 

 mediate network, n.s. the nephrostomial sinuses, branches of the ventral sinus, 

 contain the testes {t)\ botryoidal tissue {h.t.) is present; ch. chaetae; cm. 

 coecum; cr. crop; d.v. dorsal, l.v. lateral, v.v. ventral blood vessel; ect. ecto- 

 derm; gl. glands; ni.c. circular, m.l. longitudinal muscles; nep. nephridium; 

 n.c. nerve cord ; oe. oesophagus ; per. peritoneum ; s.o. sense organs. 



one another. They also communicate with the sinuses of the coelom 

 and with the capillaries of the botryoidal tissue, as has been shown 

 by careful injection. This astonishing condition is unique, but a 

 parallel may be drawn with the vertebrate in which the lymphatic 



