50 



PORIFERA 



some sponges colonies may be formed. Budding or branching is a 

 common method of reproduction in sponges. 



In the fresh water Spongilla and in some of the marine sponges, 

 the autumnal death of the adult sponge is preceded by the formation 

 of statoblasts, or gemmules. The mesogleal cells aggregate in a 

 clump, are surrounded by a firm membrane, and protected by blunt 

 spicules, called amphidiscs. These gemmules survive the winter, 



and develop into males or 

 females. From the ferti- 

 lized eggs come the summer 

 generation of sponges 

 which produce gemmules 

 and die in the fall. The 

 gemmules serve to preserve 

 the race, and to disperse it 

 as well. They may be 

 desiccated for years and 

 then grow new Spongillae 

 when the water returns. 



(2) Sexual. — Ameboid 

 wandering cells from the 

 mesoglea form eggs or 

 spermatozoa. The ferti- 

 lized eggs become flagel- 

 lated free swimming larvae, 

 then become fixed, pass 

 through a primitive gas- 

 trula stage, and finally de- 

 velop the inhalant ostia 

 and the exhalant osculum 

 of an adult. The flagel- 

 lated cells of the larva de- 

 velop into the gastral choanocytes of the adult, and the larval inner 

 cells develop into the dermal layer. 



Nervous System. — The first clearly marked neuromuscular cells 

 in the Invertebrates are found in the sponges where certain " poro- 

 cytes " are found surrounding the pores leading to the incurrent 

 canals. There is no nervous receptor present, but the porocytes 

 contract as do the Protozoa when stimulated. This may be called 

 the independent-effector stage. Although the nervous system is thus 



Fig. 19. Longitudinal section of a simple 

 sponge. 0, osculum; ip, incurrent pores. 

 (Parker and Haswell, Textbook of Zoology. 

 Courtesy of Macmillan and Co., Ltd.) 



