442 SCIENCE PROGRESS. 



sponges regarded as an archenteron, lined by a true endo- 

 derm, but by the extreme coelenterists the osculum was 

 homologised with the coelenterate mouth, and the system 

 of inhalant and exhalant canals with the coelenterate gastro- 

 vascular apparatus. It was soon established, however, 

 that the sponge larva attached itself by what had to be 

 considered, on the gastrula theory, as the blastopore, and 

 hence the homology of osculum and mouth had to be given 



up. 



It is not necessary to discuss further here the alleged 



homology of gastral cavity and stomach (archenteron), since 

 it will be seen when we discuss the development that such a 

 homology is now quite untenable. One side issue may how- 

 ever be noticed, since it is one that has contributed greatly to 

 confuse the morphology of sponges. In a simple Ascon 

 the internal cavity is lined by collar cells everywhere except 

 at the oscular rim. On the theory that the gastral cavity 

 represents the archenteron, we have in these simplest types 

 the " endoderm " represented by a uniform layer of collar 

 cells. In a Sycon, however, or any sponge above an As- 

 con, the collar cell layer is discontinuous and confined to the 

 chambers, all the rest of the canal system being lined by 

 flattened epithelium. But an archenteron cannot be imagined 

 as lined by anything else but endoderm. Hence some 

 part at least of the flattened epithelium lining the canal 

 system had to be considered as endoderm, in spite of the 

 fact that it could not be distinguished by any feature what- 

 soever from the flat " ectodermal " epithelium on the outer 

 surface of the sponge. The exact limits of the "ectoderm" 

 and "endoderm" in the interior of the canal system were 

 much disputed, but the opinion expressed by Schulze 

 (1880, p. 438) generally prevailed, according to which the 

 endoderm furnished not only the collar cells of the cham- 

 bers, but also the flat epithelium lining the excurrent 

 system from the chambers to the oscular opening, while 

 the ectoderm furnished the flat epithelium covering the 

 outer surface of the sponge and all the incurrent canals 

 up to the chamber pores. It followed therefore that an 

 endoderm consisting primitively of a uniform layer of collar 



