20 



INVERTEBRATE MORPHOLOGY. 



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

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

 acter, though in AcantJiometra it is composed of a peculiar 

 horny material termed acanthin. 



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 



Pig. 8. — Helios'pUcen'a actinota (after Haeckel from Hatschek). 



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

 Zooxantkellce 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 ai'e conferred br 

 the plant and the Eadiolarian, the coexistence constituting 

 an example of the phenomenon known as Symbiosis. 



Beproduction in the Rhizopods. — 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 



