HIRUDINEA 



133 



was the space in which the inner ends of the nephridia lay, 

 and was archieoehc in its origin. In Hirudinea this space is 

 divided into two : a series of spaces hned with an endothelium, 

 the true vascular system ; and a series of sinuses with no special 



Fio. 84. — Diagrams of transverse 

 sections : I., through CUpsine ; 

 II., through Hirudo. A. G. 

 Bourne. 



1. Alimentary canal. 



2. Dorsal sinus. 



3. Nerve cord. 



4. Ventral sinus. 



5. Lateral sinus. 



6. Ovary. 



7: Dorsal blood-vessel. 



8. Inner end of nephridium. 



9. Testis. 



1 0. Lateral blood-vessel. 



11. Nephridium. 



12. Vesicle of nephridium. 



cellular lining, and enclosing various organs of the body, the 

 coelom. These two series of spaces open into one another 

 directly in the Rhynchobdellidae, and through the hotryoidal 

 tissue in the Gnathobdellidae. In Clepsine (Fig. 84, I.) and 

 Pontdbdella the dorsal sinus contains a dorsal vessel, the ventral 

 sinus contains the nervous system, a ventral vessel, the ovaries, 

 and in Clepsine the inner ends of the nephridia. There are 

 also two lateral sinuses in which in Fontohdella a lateral vessel is 

 found. In Hirudo (Fig. 84, II.) there are a dorsal and ventral 

 sinus, and two lateral vessels. The ventral sinus encloses the 

 nervous system. The ovaries and testes lie in special sinuses. 

 The internal end of the nephridium is placed in the testis sinus 

 in those segments in which both occur. 



The alimentary canal of Leeches falls into five sections : 

 (i.) the muscular pharynx, which may possess two or more 

 commonly three jaws, and numerous glands whose secretion in 

 Hirudo serves to prevent the blood upon which the animal 

 lives from coagulating; (ii.) the oesophagus and proventri- 



