CHAPTER 1 



INTRODUCTION 



Leeches are fascinating animals, full of strange zoological paradoxes. 

 In what other group of the animal kingdom do we find such 

 features as a gut with no digestive enzymes, haemoglobin circu- 

 lating in the coelomic fluid and fertilization carried out by intro- 

 ducing the sperms through the body wall of another animal? Yet 

 such things are quite normal among leeches and are only a few 

 of their peculiarities. 



They belong to the phylum Annelida yet in many respects they 

 have advanced beyond the level of organization which we regard 

 as typical of annelids. When we compare the arthropods with the 

 annelids we notice that the arthropods usually have a smaller, 

 fixed number of segments and this has made possible a greater 

 mobility and agility. Leeches are the only major group of annelids 

 to have adopted a small, fixed number of segments for their basic 

 plan and they are certainly more agile than most other annelids. 



In oligochaetes and polychaetes there is a spacious fluid-filled 

 coelom between the gut and the body wall which serves as a hydro- 

 static skeleton. In arthropods, which have an exoskeleton to 

 support the body, the coelomic spaces are very small and the blood 

 space or haemocoel has expanded to take their place. Leeches also 

 lack the spacious fluid-filled coelom for it has become almost filled 

 with mesenchymatous packing tissue, leaving only a system of 

 narrow channels in which there is a circulation of coelomic fluid. 



The leech nervous system is built on the usual annelid pattern 

 of an anterior dorsal brain with connectives running to a paired 

 ventral nerve cord which expands in each segment to form ganglia. 

 It is characteristic of arthropods that they have more ganglia 

 crowded into the head region than have the annelids. Insects, for 

 instance, have six pairs of ganglia in the head region, of which three 

 lie dorsal to the oesophagus. Oligochaetes and polychaetes have 



