8o 



MANUAL OF ZOOLOGY. 



The Polycystina are all microscopic, and are all inhabitants 

 of the sea, having a very wide distribution. They likewise 

 extend to great depths; and one of the numerous facts of in- 

 terest brought to light by the 

 researches of the Challenger 

 Expedition, under Sir Wyville 

 Thomson, has been that large 

 areas of the sea-bottom, up to 

 the enormous depth of 4500 

 fathoms, are formed by an 

 " ooze " composed of the silic- 

 eous cases of Polycystina and 

 other Radiolarians. Similar de- 

 posits of Tertiary age are known 

 as occurring in the crust of the 

 earth in various regions. One 

 of the best known of these is 

 the " Barbadoes Earth" (fig. 

 19), which is almost wholly 

 composed of the delicate flinty 



shells of the Polycystina. The remains of Polycystina have also 

 now been detected in rocks as old as the Jurassic formation. 



III. FAM. COLLOZOA. In this family the organism is usually 

 compound, though occasionally simple. A skeleton may be 

 wholly wanting (as in the composite Collozoum\ or may exist 

 in the form of picules or of a foraminated shell. The simple 

 types always possess a mere spicular skeleton, and the same 

 is true in such forms as Sphczrozoum (fig. 20, b). On the other 



Fig. 19. Shells of Polycystina from the 

 " Barbadoes Earth ; " greatly magnified. 

 (Original.) 



Pig. 20. Morphology of Radiolaria. a Siliceous feneslrated test of Collospficera. 

 Hitxleyi ; b SpJuerozoum morum, showing cellseform bodies, compound groups of 

 spicules, and radiating pseudopodia. 



hand, in such forms as CollospJuzra (fig. 20, a) there is a 

 spheroidal fenestrated test, the skeleton thus approximating 

 in character to that of the Polycystina. The members of this 



