76 THE INVERTEBRATA 



walls, of a supplemental layer (Fig. 62 B). The nuclei, where there 

 is more than one, bear no constant relation to the chambers. 



In many species the shells are dimorphic, the two forms (Figs. 63, 

 66) being distinguished by the size and arrangement of the first formed 

 chamber, which is small in one (the microspheric form) and larger in 

 the other (megalospheric) . These forms correspond to the alternation 

 of generations in the life cycle (Fig. 66), the microspheric form, which 

 usually becomes multinucleate at an early stage, reproducing asexu- 

 ally by multiple fission, while the megalospheric form, which remains 

 uninucleate till it is about to reproduce, produces gametes. 



Most foraminifera are creeping organisms, but the Globigerinidae 

 are planktonic and have, correspondingly, vacuolated ectoplasm and 

 long slender spines on the shell. The shells of such forms, falling to 

 the bottom, form an important constituent of many deep-sea oozes. 



Allogromia (Fig. 64). Shell one-chambered, egg-shaped, pseudo- 

 chitinous. Marine and in fresh waters. 



Rhabdammina (Fig. 6 A). Shell one-chambered, straight or forked, 

 tubular, composed of foreign particles. Marine. 



Nodosaria (Fig. 6B). Shell perforate, calcareous, consisting of 

 several chambers arranged in a longitudinal row, the mouth of each 

 chamber opening into the next younger and larger. Marine. 



Polystomella (Figs. 65, 66). Shell perforate, calcareous, consisting 

 of numerous chambers, arranged in a flat spiral, and complicated as 

 follows in the details of their architecture: each whorl is equitant, 

 i.e. overlaps the previous whorl at the sides and thus hides it; the 

 mouth is replaced by a row of large pores ; backward pockets {retral 

 processes) stand along the hinder edge of each chamber; the sup- 

 plemental layer contains a system of canals filled with protoplasm. 

 Marine. The life cycle of this genus, which shows the alternation of 

 generations described above, has been followed in detail (Fig. 66). 



Nummulites (Fig. 63). As Polystomella but with more chambers. 

 Marine. Includes, besides recent forms, large fossil species in Eocene 

 limestones. 



Globigerina (Fig. 6C). Shell perforate, calcareous, chambers fewer 

 and less compact than in Polystomella, arranged in a rising (helicoid) 

 spiral, and bearing long spines. External layer of protoplasm frothy, 

 with large vacuoles by which the specific gravity is reduced. Marine, 

 pelagic. Its shells are common in oceanic oozes and in chalk. 



Order RADIOLARIA 

 Marine, planktonic Sarcodina, which have no shell but possess a 

 central capsule and usually a skeleton of spicules ; whose pseudopodia 

 are fine and radial and usually without conspicuous axial filament; 

 and the outer layer of whose protoplasm is highly vacuolated. 



