68 



SPONGES 



(a) Development ofClaihrina hhinca. — The ovum undergoes a regular 

 and total cleavage, resulting in the formation of a hollow, ciliated 

 blastula of oval form. The segmentation cavity is large, and con- 



Fio. 57. 



Uovflopiiient of Clalhriiia hlaitfa .seen as n living object with iiio<ierate inai:iiilicatioii. 1, 

 larva hv.vu in optical spction ; 2^, pupal stape of tliH first day of lixation, iiu'tjuiiorpliosis 

 complptd ; 2«, a small portion of the siinie a few hours later, showinR a distinet cpitheliiiiii on 

 the surface ; S, jiupa at the conmieiicenient of the third day after lixation, . showing the young 

 spicules and the gastral cavity beginning to form ; 4, young sponge with pores and osculum, 

 of the fifth day. f.p.c, contracted porocyte ; /l.c/i, flat epithelium ; o.ji.r, exjxinded jwrocyte; 

 osc, osculMm ; s'liic, spicule. 



tains a coagulaV)lc fluid ; its wall is composed of a single layer of 

 columnar, Hagcllated cells, with compressed or onion-shaped nuclei. 

 At one point, the future po.sterior ])ole of the larva, are a pair of 



