HIRUDINEA 



163 



The medicinal leech, owing to a great growth of this above- 

 mentioned tissue, is almost without 

 a coelomic cavity. When the body 

 is opened a narrow vessel full of 

 a red fluid is seen running along the 

 middle dorsal line above the alimentary 

 canal. This is the dorsal sinus, a 

 remnant of the true coelomic cavity; 

 a similar sinus runs along the ventral 

 surface underneath the alimentary 

 canal, which is called the ventral 

 sinus. It communicates with the dorsal 

 sinus by lateral channels which run 

 between the intestine and the posterior 

 caeca of the crop. It surrounds the 

 ventral nerve-cord, which thus seems 

 to float in blood but really lies in the 

 red coelomic fluid, and it gives off 

 lateral sinuses which surround the inner 

 openings of the nephridia. The true 

 blood-vessels comprise a vessel running 

 on each side of the body and con- 

 nected together by transverse branches 

 which run from side to side below the 

 ventral sinus. These lateral vessels 

 further supply capillaries to the 

 nephridia, alimentary canal, repro- 

 ductive organs, etc., and a very ex- 

 tensive system to the skin where the 

 haemoglobin of the blood takes up 

 oxygen. Except in Branchellion, which 

 has special gills, the respiration of 

 leeches is carried on by the skin. 



FIG. 68. View of the internal organs of Hirudo medicinalis. On the left side 

 the alimentary canal is shown, but the right half of this organ has been 

 removed to show the excretory and reproductive organs. 



1. Head with eye spots. 2. Muscular pharynx. 3. 1st diverticulum of 

 the crop. 4. llth diverticulum of the crop. 5. Stomach. 



6. Kectum. 7. Anus. 8. Cerebral ganglia. 9. Ventral nerve- 

 cord. 10. Nephridium. 11. Lateral blood-vessel. 12. Testis. 

 13. Vas deferens. 14. Prostate gland. 15. Penis. 16. Ovary. 

 17. Uterus a dilatation formed by the conjoined oviducts. 



112 



