102 B. W. PRIEST ON THE CALCAREA. 



polygonal epithelium cells, which cover the whole exterior of the 

 sponge, and line more or less the incurrent canals, the margins of 

 the cells being invisible, until treated with nitrate of silver. The 

 pores which appear on the surface are the inlialent openings, and 

 are in reality only intercellular spaces, which close themselves, 

 vanish, and are replaced by new pores, which arise by the separa- 

 tion of one cell from another. 



The endoderm, or inner membrane, consists of elongated cylin- 

 drical flagellated cells, possessing at their free ends a flagellum, 

 surrounded by a delicate hyaline marginal membrane, which is 

 derived from a prolongation of the hyaline plasma, projecting as 

 a hollow cylinder, resembling the protoplasmic collar of certain 

 Flagellata, which structure is commonly known as the collar, and 

 the cells as collared cells. It was from the presence of these 

 flagellated cells that the late Professor Clark, with some other 

 naturalists, thought that the sponges were allied to the Flagel- 

 lata, regarding them as great colonies of the same. These cells, 

 no doubt, play the part of propelling the currents of water 

 through the different channels, and likewise take up some of the 

 nutriment beneficial to the sustenance of the sponge. 



The mesoderm lies between the ectoderm and the endoderm, 

 and is probably derived from the former. It consists of a clear, 

 jelly-like matrix, in which are to be found irregularly branched or 

 spindle-shaped ameboid cells. It is in this layer we find the 

 spicules and reproductive products take their origin. The ova are 

 naked amaeboid cells, taking their origin from the cells of the 

 mesoderm, and, as far as the Syconidce are concerned, are vivi- 

 parous, remaining in the mesoderm and there undergoing develop- 

 ment ; it is very probable that the same occurs throughout the 

 Calcarea. The ova, after going through the usual segmentation, 

 first dividing in two, then four, and next eight cells, by further 

 subdivision gives rise to a solid cluster of cells, called morula, the 

 cells forming a single layer about a central cavity (blastula), which 

 normally is completely closed, but in some instances is known to 

 be open at the poles. 



In the next stage of the Calcarea an amphiblastula is formed, 

 which consists of a hollow sphere, one hemisphere formed of a 

 single layer of small, transparent, cylindrical, flagellated cells 

 (epiblast), the others of large, granular, rounded, and not flagel- 

 lated cells (hypoblast). It results from a metamorphosis of the 



