218 



CCELENTEBATA. 



In this case the canal system, in which the modifications before 

 described for each individual Sponge are repeated, becomes more 

 complex, partly through the formation of anastomoses, and partly 

 because irregular gaps and winding passages make their appearance 

 between the fused branches of the stock and form spaces which lead 

 into the ciliated cavities. 



Reproduction takes* place mainly asexually by fission and the 



production of germs or gemmules, but also 

 by the formation of ova and sperm capsules. 

 The gemmules are in the fresh-water Spongilla 

 masses of cells which are surrounded by a 

 firm, shell composed of silicious structures 

 (amphidiscs), and, like encysted Protozoa, 

 pass through a long period of rest and inac- 

 tivity. After the expiration of the cold and 

 sterile season of the year, the contents pass 

 out of the opening of the capsule and gene- 

 rally surround the latter, and with increasing 

 growth become differentiated into amoeboid 

 cells and all the essential parts of a new small 

 sponge body. Multiplication by means of 

 gemmules is also common among the marine 

 Sponges. The gemmules take their origin 

 under certain conditions as small globules 

 surrounded by a membrane. The contents 

 are essentially formed of sponge cells and 

 spicules, and, after a longer or shorter period 

 of inactivity, reach the exterior by the rupture 

 of the membrane. 



Sexual reproduction was first demonstrated 

 with certainty by Lieberkiihn for Spongilla, 

 but more recently has been shown to exist in 

 almost every group of Sponges. In most 

 fa Qya ^ spevma tozoa seem to reach 

 maturity at different times in the same Sponge. 

 The spermatozoa are needle-shaped, and lie in small spaces lined 

 with cells. The ova, like the mother cells of the spermatozoa, are 

 modified cells of the parenchyma, and are derived from cells of the 

 same tissue layer (mesoderm) in which the needles and skeletal 

 structures take their origin. The ova are naked amoeboid cells, and 

 pass into the canal system, while in the viviparous Sycons they 



PIG. i^.- 



(after O. Schmidt). 



