66 COMPARATIVE ANATOMY CHAP. 



Gelatinous substance is secreted between the ectoderm and endoderm, and into 

 this cells migrate most probably from the endoderm. Thus the connective tissue 

 mesoderm arises. At the same time radial invaginations of the endoderm vvhich lines 

 the arch-enteron are formed in the mesoderm, and grow towards the ectoderm. These 

 invaginations are rounded off and become the ciliated chambers (wL). Their com- 

 munication with the gastric cavity becomes narrowed. The ciliated chambers become 

 connected with the surface, either by the formation of pores through the external 

 membrane (in the case of chambers lying superficially), or by the formation of short 

 invaginations of the ectoderm, which finally reach the ciliated chambers. The 

 osculum arises at the aboral pole, by the lengthening and breaking through of the 

 body cavity. Sycone stage. 



According to these observations, the epithelium of the ciliated chambers, the 

 efferent canals, and the central cavity (gastric cavity) is of endodermal origin ; the 

 tesselated epithelium on the surface of the body and the epithelium of the afferent 

 canals (at least partly) of ectodermal origin. 



According to other observers, in fresh water sponges the ectoderm is thrown off 

 by the larva, and the whole adult sponge is derived from the endoderm. 



The observations of several investigators agree in establishing the fact that the 

 gastrula of the sponge attaches itself by the edges of the blastopore. The osculum 

 of the sponge therefore represents neither the blastopore of the gastrula nor the 

 mouth of the Ccelenterata. The Porifera thus appear as a laterally developed group 

 of the lower Metazoa, which do not admit of direct comparison with other Coslenterata, 

 but are only distantly related to them. 



Literature. 



O. Schmidt. Die Spongien des Adriatischen Meeres. Leipzig, 1862. Drei Supple- 



mente, 1864 bis 1868. 



The same. Grundzuge eincr Spongienfauna des atlantischen Gebietes. Leipzig, 1870. 

 Vfl^aeckel. Die Kalkschwamme. 3 Bde. Berlin, 1872. 



0. Schmidt. Die Spongienfauna des mexikanischen Meerbusens und des caraibischen 



Meeres. Jena, 1880. 



F. E. Schulze. Untersuchungen uber den Bau und die Entwickelung der Spongien, in 



Zeitschrift f. w. Zoologie. Bd. 25-35. 1876-1881. 



G. C. Vosmaer. Porifera. In Bronn's Klassen und Ordnungen des Thierreiehs. 



Leipzig, 1882. 

 N. Polejaeff. Report on the Calcarea in Chall. Exped. Rep. vol. VIII., part XXIV. 



London, 1883. 

 F. E. Schulze. Report on the Hexadinellidce. Chall. Exped. Rep. vol. XXL, 'part 



LIII. London, 1887. 

 Compare also the older works and treatises of Grant, Lieberkiihn, and Bowerbank, 



and newer investigations of Zittel, Barrois, Keller, Heider, Marshall, Lenden- 



feld, Gotte, etc. 



CLASS III. Cnidaria. 



Systematic Review. 



Sub-Class 1. Hydrozoa. 



Prototype : Hydropolyp or Hydrula. In all Hydrozoa an ectodermal oeso- 

 phagus is wanting ; the mouth leads direct into the endodermal gastric cavity. Gastral 

 filaments are wanting. The sexual products mostly arise from the ectoderm. The 

 sexes are generally separate. 



