PORIFERA 



regeneratiou of Coelenteratii and other forms, concluded that a 

 kind of " polarity " existed in tlie tissues of certain animals. 



In Oscardla lohvlaris^ the buds are transparent floating 

 bladders, derived from little prominences on the surface of the 

 sponge. Scattered in the walls of tlie bladders are flagellated 

 chambers, which ojien into tlie central cavity. The vesicular 

 nature of the buds is doubtless an adaptation, lessening their 

 specific gravity and so enaliling them to float to a distance from 

 the parent. 



GemncmlaXion.— Spongilla has already afforded us a typical 

 example of tliis process, Gemmules very similar to those of 



Siiongilla are known in a 

 few marine sponges, especi- 

 ally in Suheritcs and in 

 Ficulina. They form a 

 layer attached to the 

 surface of support of the 

 sponge — a layer which 

 may be single or double, 

 or even three or four tiers 

 deep. A micropyle is 

 sometimes present in tlie 

 spongin coat, sometimes 

 absent ; possibly its absence 

 may be correlated with 

 the piling of one layer of 

 gemmules on another, as 

 this, by covering up the 

 micropyle, would of course render it useless. Presumably when 

 a micropyle is present the living contents escape through it and 

 leave the sponge by way of the canal system (Fig. 115). 



The only case besides Spongilla in which the details of 

 development from gemmules have been traced is that of Tethyar 

 Mere microscopic examination of a Tethya in active reproduction 

 would suggest that the process was simple budding, but Maas 

 has shown that the offspring arise from groups of archaeocytes in 

 the cortex, that is to say, they are typical gemmules. As they 

 develop they migrate outwards along the radial spicule-bundles 



Fig. 115. — Gemmules of Ficulina. A, vertical 

 section of gemmules in situ ; B, vertical sec- 

 tion of upper portion of one gemmule. m, 

 Micropyle. 



1 F. E. Seluike, Zoul. Jnz. ii. 1879, p. 636. 

 Maas, Zcitschr. xciss. Zool. Ixx. 1901, p. 263. 



