15 



to their reproduction. On these grounds they have been 

 distinguished bj^ E. van Beneden,* and others, as Mesozoa. 

 Probably they are merely degraded and modified offshoots 

 from an early group of the Metazoa. 



The two great series of the Sponges, or Porifera, and the 

 Coelenterata, probably diverged from the main stem, close to 

 Gastrea, at or near the same point, or possibly they may 

 have arisen together by a short side branch representing a 

 few common ancestors after they had left the main stem. It 

 is possible, on the other hand, that the Porifera may have 

 arisen from a group of the higher Protozoa, independently 

 of the other Metazoa. f In this case, they would have no 

 close relationship with the Coelenterata. 



The Physemarial have been placed upon a separate 

 branch, close to the base of the Porifera, and arising from 

 Gastrea. This is the position assigned to them by Haeckel, 

 who described them§ as simple Gastrea-like organisms, 

 related to the lowest Sponges ; but more recent investigations 

 by Eay Lankester, || and others, have thrown very grave 

 doubts upon this interpretation of their structure, and it is 

 not improbable that they may all turn out to be merely large 

 and somewhat abnormal Foraminifera. 



The ancestral Sponges probably divided at an early period 

 in the history of the group, into two series, now represented 

 by the forms with calcareous spicules and the rest. The 

 simplest calcareous Sponges (HaeckeFs Ascones) may be 

 distinguished as Homocoela,1T from the more complex forms 

 (the Sycones, the Leucones, and the Teichones) or Hetero- 

 coela. 



* "Eecherches sur les Dicyemidea,^^ Bulletin Acad. Roy. de Belgique, 1876. 



t See Balfour, Comp. Embiyol., v. ii, p. 285. 



I Haeckel, Jen. Zeitsch., Bd. x, and Huxley's Invertebrata, p. 645. 



§ Studien zur Gastrcea-theorie, iii, Die Physemarien. 



II Quart. Journ. Micros. Sc. vol. xix, p. 476, 1879. 



1] Pol6jaeff, " Challenger' Zoological Reports, vol, viii, Part xxiv (1883). 



