126 



LEECHES 



yolk is slowly withdrawn from the centre to form a lumen sur- 

 rounded by a gut epithelium and an opening to the outside world 

 is acquired when ectodermal cells sink in to form a stomodaeum. 

 The proctodaeum is formed last of all. 



While this is in progress the mesodermal tissue beneath the 

 germinal bands is dividing up into segmental blocks, beginning at 

 the anterior end. The blocks hollow out to form the coelom and 

 spread laterally round the gut, giving rise to gut musculature and 

 longitudinal muscles of the body wall, but in addition many mesen- 

 chyme cells are budded off both inwards and outwards from the 

 walls of the coelomic sacs and septa break down to bring the 

 coelomic cavities into communication one with the other. Eventu- 

 ally the coelom becomes almost obliterated and is represented only 

 by four longitudinal channels, two lateral, one dorsal and one 

 ventral, together with a complex system of transverse channels. 

 In Glossiphoniidae blood vessels are formed mid-dorsally and 

 mid-ventrally but they do not develop a full set of segmental 

 branches, remaining instead as rather small longitudinal vessels 

 totally enclosed within the dorsal and ventral coelomic channels. 



Nephridia are formed in two parts: the funnels and glandular 

 parts are formed from nephridioblasts which are intersegmental 

 in position (Figs. 78 and 79) while the end sacs and ducts to the 



Rudiment of nephridial tubules 



Nephridioblast 

 Fig. 78 



Nephridioblast 

 Fig. 79 



Fig. 78. Longitudinal section showing the coelom, rudimentary 

 septa and nephridioblasts of Glossiphonia. After Bychowsky, 



1921. 



Fig. 79. Nephridial rudiment in Erpohdella. From Burger, 



1891. 



