566 GEPHYREA. 



or with the body cavity ! Such a duct could have no function. It is 

 intelligible either (1) 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 pro- 

 visional excretory organ a series of secondary independent segmental tubes 

 should be formed. But from Hatschek's account neither of these modes of 

 evolution can be deduced. 



Gephyrea. The Gephyrea may have three forms of excretory 

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

 position and sometimes also in structure, to the provisional ex- 

 cretory organ of Polygordius, has so far only been found in the 

 larva? of Echiurus and Bonellia. 



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

 homologous with the segmented excretory tubes of Cha^topods. 

 Their main function appears to be the transportation of the gene- 

 rative 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 pos- 

 teriorly 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 Echiurus Hatschek 

 (No. 515) finds that they are developed from the somatic mesoblast 

 of the terminal pa-.t 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 their inner extremities; and there is at first 

 but a single funnel for each vesicle. The mode of increase of the 

 funnels has not been observed, nor has it been made out how the 

 organs themselves become attached to the hind-gut. 



The provisional excretory organ of Echiurus is developed at an 

 early larval stage, and is functional during the whole of larval life. 

 It at first forms a ciliated tube on each side, placed in front of that 

 part of the larva which becomes the trunk of the adult. It opens 

 to the exterior by a fine pore on the ventral side, immediately in 

 front of one of the mesoblastic bands, and appears to he formed 

 of perforated cells. It terminates internally in a slight swelling, 

 which represents the normal internal ciliated funnel. The primi- 

 tively simple excretory organ becomes eventtmlly highly complex by 

 the formation of numerous branches, each ending in a slightly swollen 

 extremity. These branches, in the later larval stages, actually form 

 a network, and the inner end of each main branch divides into a 



