THE PORIFERA. 103 



bate. It is certain that an ordinary sponge is made up of an 

 aggregation of corpuscles, some of which have all the charac- 

 ters of Amoebce, while others are no less similar to Monads ; 

 and therefore, taking adult structure only into account, the 

 comparison of a sponge to a sort of compound Protozoon is 

 perfectly admissible, and, in the absence of other evidence, 

 would justify the location of the sponges among the Protozoa. 



But, within the last few years, the development of the 

 sponges has been carefully investigated ; and, as in so many 

 other cases, a knowledge of that process necessitates a recon- 

 sideration of the views suggested by adult structure. 



The impregnated ovum undergoes regular division ; a blas- 

 toderm is formed, consisting of two layers of cells — an epiblast 

 and a hypoblast — and the young animal has the form of a 

 deep cup, the wall of which is composed of two layers, an ec- 

 toderm and an endoderm, which proceed respectively from the 

 epiblast and hypoblast. The embryo sponge is, in fact, simi- 

 lar to the corresponding stage of a hydrozcon, and is totally 

 unlike any known condition of a protozoan. 



Beyond this early stage, however, the sponge-embryo 

 takes a line of its own, and its subsequent condition differs 

 altogether from anything known among the Ccelenterata / all 

 of which, on the other hand, present close and intimate resem- 

 blances in their future development, as in their adult structure. 



It is not long since the only sponge of the structure and 

 development of which we were accurately informed was the 

 S pong ilia fluviatilis, or fresh-water sponge, the subject of the 

 elaborate researches of Lieberkiihn and Carter. But, recently, 

 a flood of light has been thrown upon the morphology and phys- 

 iology of the marine sponges, particularly of those sponges 

 with calcareous skeletons, which are termed Calcispongiw, 

 by Lieberkiihn, Oscar Schmidt, and especially Haeckel. It 

 has become clear that Spongilla is a somewhat aberrant 

 form, and that the fundamental type of Poriferal organization 

 is to be sought among the Calcispongia?. In the least com- 

 plicated of the calcareous sponges, the body has the form of 

 a cup, and is attached by its closed extremity. The open ex- 

 tremity is the osculum, and leads directly into the spacious 

 ventricithis, or cavity of the cup. The comparatively thin 

 wall of the cup is composed of two layers, readily distinguish- 

 able by their structure — the outer is the ectoderm, the in- 

 ner the endoderm. The ectoderm is a transparent, slightly 

 granular, gelatinous mass in which the nuclei are scattered, but 

 which, in the unaltered state, shows no trace of the primitive 



