,9o8.] EXCRETORY ORGANS OF METAZOA. 607 



(Collembola), or when this fails there is rapid death of the indi- 

 vidual from poisoning of the intestinal tract (Ectoprocta). 



2. Homologies of the Preceding Types. 

 The entoblastic type ( 14) is sui generis and not related to the 

 others. Types 12 (peritoneal glands) and 13 (retroperitoneal dif- 

 ferentiations) are so generalized in both structure and function, 

 that it is hardly advisable to attempt to draw homologies between 

 them; and the same holds for types i (ectoblastic skin glands), 2 

 (ectoblastic vesicles), 5 (scattered mesectoblastic cells) and 6 

 (mesectoblastic vesicles). There remain then for consideration all 

 those distinctly tubular organs, nephridia proper, into the composi- 

 tion of which entoblast does not enter. ^* The earliest and most 

 uniform of these are those of type 3, ectoblastic invaginations ter- 

 minating in flame cells, which are referable, as argued by Lang, to 

 still simpler skin glands. Type 4, ectoblastic invaginations like 3 

 but without cilia, are essentially similar ; for no one would hesitate 

 to homologize the mid-gut of the Turbellaria and the Insects, though 

 the former is ciliated and the latter is not ; therefore one should not 

 object to drawing homology between the water vascular system of 

 the former and the Malpighian vessels of the latter. The lack of 

 cilia is not a characteristic merely of these vessels, it marks all the 

 tissues of the Insects. The only differences between types 3 and 4 

 is the lack of cilia in the latter, and this is a difference that is of 

 little homological importance, a merely histological character. And 

 essentially similar to both of these is type 7, tubes of mesectoblastic 

 origin ; they do not come immediately from the ectoblast, but from 

 tissue of ectoblastic derivation which is but a step removed. These 

 three types, accordingly, 3, 4 and 7 are anatomically and embryolog- 

 ically essentially alike, they are to be considered homologous ; they 

 stand in no relation to the ccelom, never conduct the genital prod- 



" The term nephridium has been used very variously since its coinage 

 by Lankester (1877). It might be well to limit it in the future to tubular 

 excretory organs not containing entoblast. In the descriptive part of the 

 paper I have discussed special homologies of excretory organs within the 

 same group, such as relations of embryonic to adult nephridia, of mega- 

 nephridia and plectonephridia, homologies of tracheae, etc. ; these need not 

 be repeated here. 



