686 GEPHYREA. 



nent excretory tubes is diagrammatically represented in fig. 

 384 C and D. 



The provisional excretory organ atrophies during larval life. 



If Hatschek's account of the development of the excretory system of 

 Polygordius is correct, it is clear that important secondary modifications 

 must have taken place in it, because his description implies that there sprouts 

 from the anterior excretory organ, while it has its own external opening, a 

 posterior duct, which does not communicate either with the exterior or with 

 the body-cavity! Such a duct could have no function. It is intelligible 

 either (i) that the anterior excretory organ should lead into a longitudinal 

 duct, opening posteriorly ; that then a series of secondary openings into the 

 body-cavity should attach themselves to this, that for each internal opening 

 an external should subsequently arise, and the whole break up into separate 

 tubes; or (2) that behind an anterior provisional excretory organ a series of 

 secondary independent segmental tubes should be formed. But from Hat- 

 schek's account neither of these modes of evolution can be deduced. 



Gephyrea. The Gephyrea may have three forms of excre- 

 tory organs, two of which are found in the adult, and one, 

 similar in position and sometimes also in structure, to the 

 provisional excretory organ of Polygordius, has so far only been 

 found in the larvae of Echiurus and Bonellia. 



In all the Gephyrea the so-called 'brown tubes' are 

 apparently homologous with the segmented excretory tubes of 

 Chaetopods. Their main function appears to be the transport- 

 ation of the generative products to the exterior. There is but a 

 single highly modified tube in Bonellia, forming the oviduct and 

 uterus ; a pair of tubes in the Gephyrea inermia, and two or 

 three pairs in most Gephyrea armata, except Bonellia. Their 

 development has not been studied. 



In the Gephyrea armata there is always present a pair of 

 posteriorly placed excretory organs, opening in the adult into 

 the anal extremity of the alimentary tract, and provided with 

 numerous ciliated peritoneal funnels. These organs were stated 

 by Spengel to arise in Bonellia as outgrowths of the gut; but in 

 Echiums Hatschek (No. 515) finds that they are developed from 

 the somatic mesoblast of the terminal part of the trunk. They 

 soon become hollow, and after attaching themselves to the 

 epiblast on each side of the anus, acquire external openings. 

 They are not at first provided with peritoneal funnels, but these 

 parts of the organs become developed from a ring of cells at 



