ORIGIN OP METAMERIC SEGMENTATION. 97 



the Capitellidae, in which the excretory organs end blindly 

 against the ectoderm ; their products, therefore, must pass to 

 the exterior in some such way as I have suggested. If my 

 suggestion be correct, it follows that the excretory organs were 

 in their origin not specially organs for the excretion of nitro- 

 genous waste products (each cell of the body being in close 

 relation to the exterior did this itself) but for the riddance of 

 the undigested and solid excretory products ; and also that the 

 excretory process was in its origin an intra-cellular process, i. e. 

 temporary passages (amoeba) were formed in the cells, through 

 which the solid products passed to the exterior. This latter 

 deduction is supported by the fact that in the higher animals 

 the first formed excretory organs of the larva (Hatschek, 

 Polygordius; Caldwell, Phoronis) have the form of delicate 

 ducts attached to and opening through the ectoderm and 

 ending in the body cavity, each in a simple cell ; i. e. they are 

 blind internally, and the excretory products in the body cavity 

 must pass through the cell to get to the exterior. 



Whatever view may be held as to the origin of the pores, 

 the fact of their existence in the Diploblastica is undisputable. 



At first irregularly arranged (a condition retained in Actino- 

 zoa, but more markedly in Sponges), they soon acquired a 

 regular arrangement (Medusae), and on the diflferentiation of 

 the alimentary cavity into a digestive part (gut proper), 

 and a circulatory and excretory part (coelom), they remained 

 in connection with the coelom, which latter became again 

 differentiated into parts purely excretory and connected with 

 the pores (nephridia), and into the general vascular space for 

 the circulation of the nutritive fluids passed into it from the 

 endoderm cells. 



Turning to the development of the excretory organs of the 

 higher animals, we find that in the Vertebrata they arise 

 as special parts ^ (not mere outgrowths) of the coelom, and I 

 have no doubt that this will be soon shown to be the case for 

 the development of the Invertebrate excretory organs. 



I Sedgwick, " Development of Kidney, &c.," ' Quart. Journ. of Mic. Sci.,' 

 vol. XX, 1880. 



