90 ESSENTIALS OF ZOOLOGY 



cavity into which a pair of protonephridia are thrust from the 

 .ectoderm as excretory organs. They are sometimes branched 

 into two, and the branches end blindly, as usual, in solenocytes. 

 The secondary mesoderm and the coelom appear only during 

 the conversion of the larva into the worm, and the nephridia 

 are developed then : they are ectodermal in origin and, as in 

 Lumbricus, are put into communication with the coelom. In 

 some Polychaets they act also as gonoducts. 



The Podectathenta are very similar in their history, but 

 in them the coelom remains simple and the space is primarily 

 a gonocoel. Protonephridia are developed in relation to the 

 schizocoel in front of the coelom, and appear in all cases to be 

 converted into the nephridia of the adult, where they act also 

 as gonoducts. 



