68 



SPONGES 



(a) Development of Clathrina blanca. 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. 



Development of Clathrina blanca seen as a living object with moderate magnification. 1, 

 larva seen in optical section ; 2^, pupal stage of the first day of fixation, metamorphosis 

 complete ; 2S, a small portion of the same a few hours later, showing a distinct epithelium on 

 the surface ; 3, pupa at the commencement of the third day after fixation, showing the young 

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

 of the fifth day. c.p.c, contracted porocyte ; fl.ep, flat epithelium ; o.p.c, expanded porocyte ; 

 osc, osculum ; spic, spicule. 



tains a coagulable fluid ; its wall is composed of a single layer of 

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

 At one point, the future posterior pole of the larva, are a pair of 



