2-M COMPARATIVE ZOOLOGY. 



mals, which are constantly falling upon it (Globigerina- 

 ooze). Their remains constitute a great proportion of the 

 so-called sand-banks which block up many harbors. Yet 

 they are the descendants of an ancestry still more prolific ; 

 for the Foraminifera are among the most important rock- 

 building animals. The chalk-cliffs of England, the building- 

 stone of Paris, and the blocks in the Pyramids of Egypt 

 are largely composed of extinct Foraminifers. Forami- 

 nifera are both marine and fresh-water, chiefly marine. 



A Radiolarian differs from a Foraminifer in secreting a 

 siliceous, instead of a calcareous, shell, studded with radi- 

 ating spines; and the central part of the body is made up 

 of a colony of cells, and surrounded by a strong membrane. 

 They are also more minute, but as widely diffused. They 

 enter largely into the formation of some strata of the 

 earth's crust, and abound especially in the rocks of Barba- 

 does and at Richmond, Ya. The living forms are mostly 

 marine, but some are fresh-water. 



CLASS III. Gregarinida. 



The Gregarinse, discovered by Dufour in 1828, are 

 among the simplest animal forms of which we have any 

 knowledge. The only organ is a nucleus, suspended in 

 extremely mobile protoplasm which is covered by a cuti- 

 cle ; and the most conspicuous signs of life are the con- 



Fi. 186. Gregarina gigantea, highly magnified: a, nucleus. 



traction and lengthening of the worm-like body. They 

 feed by absorption, and are all parasites, living in the ali- 

 mentary canal of higher animals; as in the Cockroach, 

 Earth-worm, and Lobster. The name is derived from the 

 fact that they occur in large numbers crowded together. 



