TYPE COSLENTERA. 75 



productive elements, ova and spermatozoa, differentiating 

 from mesogloaal cells. Many Sponges are hermaphrodite, the 

 spermatozoa developing usually somewhat in advance of the 

 ova, but some forms seem to have separate sexes. The ova 

 are fertilized while still within the tissues of the parent and 

 undergo a portion of their development there, later breaking 

 through into a canal and so passing to the exterior as a cili- 

 ated free-swimming structure. The segmentation of the 

 ovum in typical cases results in the formation of a blastnla, 

 which becomes converted into a solid ciliated sterrula by 

 immigration. After swimming about for a time the sterrula 

 loses its cilia and settles doAvu, a cavity appearing in its in- 

 terior, which later, in forms which possess ciliated chambers, 

 gives rise to these structures as a series of pouches in connec- 

 tion with which canals arise. The pores and the osculum 

 finally break through, imaginations from the exterior giving 

 rise to the former. 



In some forms, however, the blastula stage undergoes 

 imagination, usually of a rather peculiar form, in which case 

 a gastrula results instead of a sterrula. The gastrula settles 

 down and becomes fixed by the pole at which the blastopore 

 occurs, the further development being similar to that found 

 in the immigration types. 



In addition to the sexual method most Sponges also pos- 

 sess the power of non-sexual reproduction, dependent on their 

 capabilities for regeneration. A detached portion of a Sponge 

 will, under favorable conditions, regenerate into a new indi- 

 vidual, and this power has been applied to the artificial re- 

 production of the commercial Sponges. In some forms in 

 addition to this a process of internal budding occurs, a num- 

 ber of the mesogloeal cells aggregating together and develop- 

 ing into an oval, ciliated, sterrula-like structure which, leaving 

 the parent, develops into an adult Sponge (Esperella). In the 

 fresh-water Spongilla this process is carried to the greatest 

 extent, and towards the approach of winter in temperate 

 latitudes completely replaces the sexual method. The in- 

 ternal buds of Spongilla are known as gemmules and are 

 especially adapted for tiding the species over unfavorable 

 conditions, such as cold or dryness, which the vegetative in- 



