I 



PROTOZOA 



^5 



tropisms are indicated: (i) Photo tropism, or heliotropism — reaction 

 to light. (2) Geotropism — reaction to gravity. (3) Rheotropism 

 — reaction to currents (stream pressure). (4) Thermotropism — re- 

 action to heat. (5) Thigmotropism — reaction to touch or contact. 

 (6) Chemotropism — reaction to a chemical. (7) Galvanotropism 

 — reaction to electrical currents. 



In general we find that for any animal there exists an optimum 

 attracting stimulus, which we may term positive tropism (or taxis) ^ 

 and a negative stimulus, usually the more powerful one. For 

 example, the ameba will be positively phototropic to a certain light, 

 but negatively phototropic to one of greater intensity. 



Orders of Sarcodina. Order i. — Lobosa — (Ameba) soft jelly-like 

 — 5-200 ii. in diameter. They are found in pools of stagnant water. 

 Each species assumes its characteristic shape (Fig. 3 A). They are 

 full of granules and have one or more nuclei and a contractile vacu- 

 ole. They reproduce by simple fission, by sporulation, and rarely 

 by conjugation. 



Order 2. — Foraminifera. They have a test or shell full of 

 openings, through which project filose pseudopodia. They are 

 chiefly marine, varying in size from microscopic to two inches in 

 diameter. Their shells are calcareous, siliceous and chitinous 

 (horny). There are 120 species of Foraminifera in English chalk 

 clifi^s. The Norfolk chalk measures are 1,450 feet thick. Globi- 

 gerina ooze forms gray chalk which is deposited on the bottom of the 

 ocean to depths of 2,500 fathoms. They reproduce by motile swarm 

 spores and by binary fission. Sometimes young with shells are 

 formed in the terminal chamber of the adult. The nummulites, the 

 largest of the foraminifera, are as large as a silver dollar. The 

 limestone pyramids of Egypt are full of nummulites. 



Order J. — Heliozoa are mostly found in fresh water. They have 

 fine stiff radiating pseudopodia. Some have skeletons of delicate 

 siliceous spicules. Some species are colonial. Reproduction is by 

 fission^ spore-Jorrnation and by conjugation (Fig. 4). 



Order 4. — Radiolaria are all marine. They differ from Heliozoa 

 in having a much more elaborate skeleton of siliceous or other 

 mineral substance. They have a central capsule surrounding the 

 nucleus. They are united to form colonies of various shapes in some 

 groups. Fossil radiolaria are found in slate, flint, chalk and deep 

 sea deposits. 



