NATURE AND AFFINITIES OF THE SPONGES. 181 



constituents. Sooner or later, the segmentation cavity becomes filled up 

 with these metamorphosed, or, as Oscar Schmidt has denominated them, 

 "wandering cells," which breaking their way through the posterior pole, 

 ultimately appear as a projecting heap in this region, and thus convey to 

 the entire organism an aspect closely corresponding with the normal 

 amphiblastuloid type. It is through the medium of these projecting meta- 

 morphosed cells that the young sponge becomes fixed to its selected 

 fulcrum of support, and that the further development onward to the charac- 

 teristic adult sponge-stock is initiated. Some of the most characteristic 

 representations of the phenomena last described, as given by Oscar Schmidt, 

 will be found reproduced at PL IX. Figs. 36-38. The remarkable and 

 important data, first elicited by the last-named authority, have been entirely 

 confirmed by the later researches of Metschnikoff, * in connection with both 

 Ascett.i primordialis and various other widely separated sponge-forms. Pro- 

 ceeding still further, however, this investigator maintains that in these types 

 not only the so-called endodermic, but special mesodermic (spiculiferous) 

 elements are likewise produced by a similar developmental process. 



The task of reconciling all of the various developmental formulae now 

 enumerated as exhibited by diverse, or, it may be, by even a single sponge 

 type, with one or any of those that typically characterize the Metazoic 

 embryo, -is obviously almost hopeless : it now remains to be seen whether 

 or not more substantial progress in the interpretation of the affinities of 

 the sponges by means of the so-called ciliated larvae can be accomplished 

 in a totally distinct direction. Presuming that the account given by 

 F. E. Schulze of Sycandra raphanus represented the typical and constant 

 form and process of development though practically such is very far from 

 the case Mr. T. M. Balfour, one of our leading authorities in this country 

 on embryologic matters, has quite recently, January 1879, contributed to 

 the ' Quarterly Journal of Microscopical Science ' the results of his own 

 analysis of the evidence so far adduced. The verdict he arrives at, while 

 not proceeding to the full length advocated by the author of this volume, is 

 noteworthy for its marked bias in a similar direction, and for its deviation 

 in an essential manner from that Metazoic interpretation hitherto most 

 generally acquiesced in by Continental biologists. 



Mr. Balfour's expressed views in this connection are so important as to 

 demand quotation in extenso. After enumerating the chief features of the 

 developmental history of Sycandra raphanus as last reported by F. E. 

 Schulze, he thus proceeds : 



" The first point in the development of Sycandra which deserves notice is the 

 character of the free-swimming larva. The peculiar larval form, with one half of 

 the body composed of amoeboid granular cells, and the other of clear ciliated cells, 

 is nearly constant among the Calcispongiae, and widely distributed in a modified 

 form amongst the Fibrospongias and Myxospongise. Does this larva retain the 

 characters of an ancestral type of the Spongida, and if so, what does its form mean ? 



'Zeitschrift Wissenschaftliche Zoologie,' Bd. xxxii., 1879. 



