74 



THE PROTOZOA' 



between the youngest or external chambers and the oldest or internal. 

 Hence there are morphological and physiological grounds for sepa- 

 rating the Reticulariida into Perforina and Imperforina. It frequently 

 happens that the central or original chamber varies in size in the 

 same species, being large (inegalospheric) in some individuals, and 

 small (inicrospheric) in others (Fig. 38). While the relations of these 

 two forms have been much discussed, no satisfactory conclusion has 

 yet been reached. Lister ('95) regards the case as one of alternation 

 of generations in which spores from individuals A conjugate and form 

 individuals of the type B, while the latter develops spores which grow 

 into the f orm A again. The conjugation of swarmers in these dimor- 

 phic types is a matter of inference rather than of observation, for the 

 process has never been seen. 



Fig. 38. — Megalospheric (A) and microspheric (B) shells of Biloculina depressa Lam. 



[SCHLUMBERGER.] 

 The dimorphism is shown by the central chamber c. 



Among the Heliozoa and Radiolaria, shell formation is of a 

 somewhat different type, consisting of the deposition of spicules 

 and rays rather than a continuous layer of material forming a 

 compact coating. Even naked forms of Heliozoa, such as Actino- 

 spharium, secrete these spicules at certain times for the purpose 

 of encystment, while others have them in greater or less numbers 

 throughout life. Isolated spicules are usually retained by a gelat- 

 inous mantle, which covers the entire animal (Nuclearia, Acti- 

 nolopkus, etc.). These spicules are usually curved or straight rods, 



