PROTOZOA I SPONGIDA. 69 



(sometimes called " cellaeform bodies ") with distinct nuclei, 

 surrounded by a zone of siliceous spicules, the whole being 

 embedded in a common gelatinous matrix. The centre of 

 the mass is vacuolated, sometimes to such an extent that it 

 becomes a hollow sphere. 



In Collosphcera, the spherical body which is very like that 

 of the preceding form is enclosed in a transparent siliceous 



Fig. n. Morphology of Radiolaria. a Siliceous fenestrated test of Collosphara 

 Hiixleyi j b Thaiassicolla mowtm, showing cellseform 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. n, a). 



Thaiassicolla differs little from either of the above in fun- 

 damental structure, but it contains a number of compound 

 siliceous spicules embedded in its ectosarc (fig. n, b). 



CHAPTER V. 



SPONGIDA. 



ORDER 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 " sar code-bodies, destitute of 



