REPRODUCTION AND DEVELOPMENT 133 



the circular muscles and the definitive epidermis; two rows of 

 mesodermal cells destined to form the somites ; an endodermal sac 

 which will eventually form the crop and intestine of the adult leech. 

 Subsequent development is by way of a larva equipped with a 

 ciliated larval ectoderm, a temporary larval pharynx and proton- 

 ephridia. At metamorphosis these are lost, just as in the 

 gnathobdellids. 



The Piscicolidae occupy in respect of their development a 

 position intermediate between the Glossiphoniidae and the 

 Gnathobdellae. They share with the former an unspecialized 

 pattern of cleavage in which all four quadrants contribute to the 

 final animal but resemble the latter in possessing a larval stage with 

 larval organs which are shed at metamorphosis. In the Glossi- 

 phoniidae the eggs carry a large store of yolk which enables them 

 to complete their development without absorbing nutriment from 

 the fluid in the cocoon, while in the Piscicolidae and 

 Gnathobdellae there is little yolk and there is a larval stage 

 adapted to the absorption of nutriment from the fluid in the cocoon. 



8. Phylogenetic Considerations 



Whereas in polychaetes external fertilization and a free swim- 

 ming larva is normal, most earthworms and leeches have evolved 

 a habit of enclosing the fertilized eggs in a thick-walled cocoon 

 filled with nutritive fluid. This frees the parent from the necessity 

 of providing the eggs with a large supply of yolk, for nutriment 

 can be absorbed as development proceeds. Schmidt (1944) con- 

 siders that the habit has arisen by parallel evolution in the 

 terrestrial oligochaetes, the gnathobdellid and the piscicolid 

 leeches. In each case this method of rearing the eggs has resulted 

 in a number of embryonic adaptations, particularly in the forma- 

 tion of a larval pharynx, an ectodermal ciliary apparatus for 

 passing nutriment into the gut rudiment and larval nephridia. 

 The fact that the adaptations are arrived at in a different way in 

 each group points to an independent origin in each. The glossi- 

 phoniid leeches lack these adaptations and we may consider 

 whether this state has been achieved by simplification or is 

 primitive. In favour of the first view is the presence of a ciliated 

 larva in the polychaetes. This might suggest that while the other 



