PROTOZOA 89 



to capture food, but some, such as Globigerina and some twenty 

 other modern species, have taken to a pelagic existence, that is, 

 they have become planktonic. 



Reproduction in the Foraminifera is both sexual and asexual. 

 The sexually produced test results from the conjugation of zoo- 

 spores. It has a microspheric first chamber (proloculum). 

 With increase in size and in number of chambers the nucleus 

 also divides, often resulting in a greater number of nuclei than 

 chambers. In the adult, each nucleus takes some of the proto- 

 plasm and forms a megalospheric first chamber (proloculum) 

 larger than the microspheric ; this develops into an adult test 

 similar to the microspheric test, but smaller. In this adult form 

 there is a single nucleus which moves forward so as to occupy 

 the middle chamber numerically ; this finally divides, each 

 portion taking some of the protoplasm ; a further division results 

 in zoospores, or it may first continue for some generations repro- 

 ducing asexually. The life cycle, consisting of an alternation of 

 generation, is thus completed (14). 



Most Foraminifera are marine ; a few of the simpler forms live 

 in fresh water. They are now found everywhere in all seas 

 (seldom below 2500 fathoms, since by the time such depths are 

 reached the tests are dissolved). They flourish best in waters 

 free from sediment and are hence much less abundant in estuaries 

 and at the mouths of rivers than elsewhere. 



Foraminifera did not become rock builders until the Missis- 

 sippian period, Fusulina being one of the earliest to form rock- 

 masses. The chalk of the upper Cretaceous of Europe and of 

 North America (Niobrara, Austin and Rotten limestone forma- 

 tions) is largely composed of Foraminifera ; accompanying these 

 are sponge spicules, sea-urchins and shallow-water mollusks, 

 indicating shallow seas for the growth of these ancient forami- 

 niferal — or globigerina — oozes. 



Foraminifera occur from the Ordovician to the present. Ow- 

 ing to the resemblance of their convoluted chambered shells to 

 those of Nautilus, they were first classed with the Cephalopoda. 



