20 



IN VERTEBRA TE MORPHOL OGY. 



spicules, while in Thalassicolla, already alluded to (Fig. 7), it 

 is entirely absent. As stated, it is usually siliceous in char- 

 acter, though in Acanthometra it is composed of a peculiar 

 horny material termed acauthiu. 



Scattered through the protoplasm of the Eadiolarians 

 there are usually to be seen numbers of small yellowish 

 bodies long known as the "yellow cells." They are not con- 

 stant, however, individuals of any species frequently being 

 destitute of them, a peculiarity due to the "yellow cells" 

 not being really constituent parts of the Eadiolarian, but 





FlG. 8. Heliosphcera actinota (after HAECKEL from HATSCHEK). 



foreign bodies, in fact unicellular plants, for which the term 

 Zooxanthellce has been proposed. They cannot be consid- 

 ered parasites, since they do not appear to exist at the ex- 

 pense of the host, but, on the contrary, their presence seems 

 actually to be beneficial. Mutual benefits are conferred by 

 the plant and the Radiolarian, the coexistence constituting 

 an example of the phenomenon known as Symbiosis. 



Reproduction in the Rluzopods. Throughout all the groups 

 the simplest form of reproduction, fission, is probably preva- 

 lent (Fig. 9), though it is not yet definitely known to occur in 



