50 



ZOOLOGY 



bEOT. 



from a cyst in having an aperture at one end to allow of the 

 protrusion of pseudopods from a small naked area. This is exactly 

 what we find in Arcclla and its allies (Fig. 32, A-c), in which the 

 shell is chitinoid. A different kind of shell is found in Dijfflugia 

 (d), which secretes a gelatinous coating to which minute sand- 

 grains and other foreign particles become attached. 



Order 2. — Foraminifera 



General Structure. — The members of this order differ from 

 the Lobosa in the fact that their pseudopods are long and delicate 

 and unite to form networks ; moreover, with few exceptions, they 

 agree with Arcella and its allies in possessing a shell. In the 

 majority of cases this shell is formed of calcium carbonate. 



One of the simplest members of the group is Microgromia (Fig. 

 33). It consists of a protoplasmic body (.B), with a single nucleus 



Fia. 33. — Microgromia socialis. A, entire colony ; B, single zooid ; C, zooid which 

 has undergone binary fission, with one of the daughter-cells creeping out of the shell ; 

 D, flageUuliv; c. vac. contractile vacuole ; nu. nucleus; ih. shell. (From Biitschli's Protozoa^ 

 after Hertwig and Lesser.) 



{nu.) and contractile vacuole (r. vac), enclosed in a chitinoid cell- 

 wall or shell {sh.) with an aperture at one end through which the 

 protoplasm protrudes and is produced into delicate radiating 

 pseudopods. The animal multiplies by binary fission, and the 

 individuals or zooicls thus produced remain united in larger or 

 smaller clusters, or cell-colonies (A). Sometimes the cell-body of a 

 zooid divides and one of the daughter-cells creeps out of the cell- 

 wall (C), and, after moving about for a time like an Amceba, draws 

 in its pseudopods, assumes an oval form, and sends out two 

 flagella by means of which it is propelled through the water (D). 

 We shall find other instances in which the young of a Rhizopod is 



