Hyatt.] 106 [March 5, 



those of Dr. Sedgwick's (Quart. Journ. Micr. Sci., 1884, p. 63), but 

 he seems to regard the pores and the formation of tubes as the 

 result of the outgrowth of the diverticula, whereas among sponges 

 the facts all seem to imply that the pores were antecedent and 

 possibly had an influence in leading to the development of tubes 

 and of coelomic outgrowths from the archenteron. This very 

 interesting and remarkable treatise, however, shows plainly that 

 the author saw the importance of the pores in the Porifera, and 

 takes these and the internal cavity as a starting point for his 

 homologies. He considers the nephridia of the higher animals 

 and the excretory tubes of Medusae to correspond with the pores 

 in sponges, and to have arisen during the differentiation of the 

 gastro-vascular system from some such primitive forms as they 

 now have in those animals. These tubes viewed as connected 

 with the water vascular system suggest comparison with the 

 excretory pores of the gastro-vascular system in the Medusae and 

 Ctenophorae, and seem to support fully the homologies of Sedg- 

 wick (ibid. p. 63), but here again the limits of the endoderm are 

 the indications of the extent to which the homology may be car- 

 ried. If, as maintained by Sedgwick, the nephridia are generally 

 formed directly from the coelomic sacs in Vertebral a and 

 Invertebrata, the ectoderm not entering into their composition, 

 and they may be considered as breaking through the ectoderm by 

 resorption, it seems evident that the true homologues of these 

 tubes in Porifera are only those parts of the incurrent tubes, which 

 are also formed by the endoderm. 



When viewed, however, as connected with the formation of 

 pits and ingrowths of the ectoderm, this theory harmonizes with 

 Dr. Sedgwick's hypothesis of the origin of the trachea only by 

 admitting that the invaginations are results and not themselves 

 primitive forms. The tracheal tubes, for example, are regarded 

 by Sedgwick (ibid. p. 66) as distinct in origin from pores or tubes 

 and as proceeding from pits in the ectoderm and also as aris- 

 ing primarily as water breathing organs. The poriferan theory 

 carries this view back one step more to the possible origin of the 

 pits as subsequent separations of the outer or ectodermic elements 

 of the tubes, which arose in connection with the primitive forms or 

 the gastro-vascular apparatus ; thus, though denying the origin of 

 the tracheae from pits in the skin of some primitive form, this 



