D.— ZOOLOGY. 71 



than this ; if so, in view of the fact that Enterozoa arose later than sponges, 

 it might seem easiest to guess that Enterozoa arose from Porifcra vera, 

 and that the blastopore which persists as an anus in Echinoderms re- 

 capitulates the osculum of a Sycon. This does not oppose the view that 

 the keyhole blastopore of molluscs or Peripatus may recapitulate the 

 siphonoglyphs of a coral 38 ; but I suggest that if there be recapitulation 

 in the gastrulae, it is recapitulation of a comparatively high stage of 

 development (as Sedgwick supposed), and that the cavity into which 

 the blastopore opens is in either case a cavity which was lined with the 

 embryologically outer layer of the ancestor recapitulated. Gill-slits, the 

 posterior position of the blastopore, enteric diverticula, the calcareous 

 spicules of Echinoderms and the embryology of the Ctenophora 39 offer 

 great temptations, but I will not waste more of your time on the easy 

 elaboration of the hypothesis that Metazoa, or some of them, are descended 

 from sponges. 



When Metazoa arose, the world contained Protista, Sponges and 

 Algae. It seems more easy to imagine the evolutionary steps which would 

 convert a Sycon into an Enterozoon than those which would build one 

 up out of unicellular Protozoa, or convert into a beast of prey the green 

 and innocent Volvox. That must be decided by those who study the 

 Metazoa and their embryology. As a student of sponges I entirely reject 

 the view that there is any common ancestor, above the flagellate monad, 

 from which the two branches of Parazoa and Metazoa have diverged. If 

 the hypothesis be acceptable that Parazoa were parents to Metazoa, 

 the word ' animals ' may still be used phylogenetically to include alike 

 the Enterozoa and the sponges from which they sprang. If those who 

 study Metazoa reject this hypothesis, the Mickophaga must be recognised 

 as constituting a third kingdom of multicellular organisms, specialised 

 for the intracellular digestion of living organisms under 5[i in diameter. 



APPENDIX A. 

 Salinity of the Ocean. 



Joly has given his opinion, as a chemist, that the original boiling and 

 superheated ocean only dissolved from the magma one-ninth the percentage 

 of chlorides which our ocean now possesses. Professor Joly assumes that, 

 ever since that primeval ocean, sodium brought down by the rivers has 

 been steadily accumulating in the sea and progressively increasing its 

 salinity. This assumption the man in the street may criticise. 



About half the sodium in the rivers comes from sedimentary rocks: 

 where did the sedimentary rocks get it from, if not from the sea under 

 which they were formed ? When a portion of the ocean is landlocked and 

 evaporated, as began 5000 years ago over the Caspian plains, the distilled 



38 Adam Sedgwick : On the origin of segmented animals and the relation of the 

 mouth and arms to the mouth of the Coelenlerata : in Proc. Camb. Phil. Soc. vol. v, 

 1884. The historian may care to know that Sedgwick gave this theory to us in his 

 lectures in the Michaelmas term of 1882, 1 think two months before he read his paper 

 on Peripatus at the Royal Society. 



39 Besides the segmentation of the ovum in Berde, Idi/ia, &c, see Mortensen's account 

 of Tjalfjellia, 1912, quoted by MacBride, Enc. Brit. vol. 30, p. 973. 



