54 MANUAL OF ZOOLOGY. 



imbedded in a common gelatinous matrix. The centre of the' 

 mass is vacuolated, sometimes to such an extent that it be- 

 comes a hollow sphere. 



In Collosphcera the spherical body which is very like that 

 of the preceding form is enclosed in a transparent siliceous 



Fig. 8. Morphology of Radiolaria. a. Siliceous fenestrated test of Col- 

 losphcera Huxleyi. b. Thalassicolla morum, showing cellasforrn bodies, 

 compound groups of spicules, and radiating pseudopodia. 



envelope, which is perforated by numerous rounded apertures 

 or ' fenestrae.' This form, therefore, approaches very closely 

 to the Polycystina, especially to those in which the foramina 

 are so large that the test is reduced to a mere reticulate frame- 

 work (fig. 8 a). 



Thalassicolla differs little from either of the above in 

 fundamental structure, but it contains a number of compound 

 siliceous spicules imbedded in its ectosarc (fig. 8 &). 



CHAPTER V. 



SPONGIDA. 



THE true nature of sponges has long been a matter of dispute, 

 but they are now almost universally referred to the animal 

 kingdom, and placed either in or near the Rhizopoda. Some 

 observers still maintain the vegetable nature of sponges, but 

 this opinion has no real grounds for its support, and is chiefly 

 founded upon loose analogies and upon a certain similarity in 

 outward form. 



The Spongida may be defined as * sarcode-lodies, destitute of 

 a mouth, and united into a composite mass, which is traversed by 

 canals opening on the surface, and is almost always supported 

 by a framework of horny fibres, or of siliceous or calcareous 

 spicula.' (Allman.) 



