134 SYSTEMATIC ZOOLOGY FOR TEACHERS. 



known, without fertilization from any other individual, although something of 

 the kind may take place. The progressive life of the species permanently resides 

 within the nucleus, and it is a significant fact, and one which should be 

 kept constantly in mind, that by the early division of the nucleus^ a portion 

 of this progressive life is insured to each half of the amoeba. In the encyst- 

 ment of the amoeba we see a provision for the perpetuation of the race under 

 conditions adverse for ordinary existence. 



RADIOL A MA. 



These are similar, in some ways, to the foraminifera, but are provided 

 with a silicious shell or test3, perforated with minute openings from which 

 the pseudopodia are thrust out. The remains of the radiolaria formed immense 

 beds of rock in the Tertiary Age, and they occur in the diatomacous rock at 

 Richmond, Va., and in that vicinity. In the Nicobar Islands they form a 

 deposit between eleven hundred and two thousand feet in thickness, and 

 there is another celebrated deposit in the interior of Barbados, West Indies. 



HELIOZOA, OR SUN ANIMALCULES. 

 These are beautiful Rhizapods and occur in fresh water. Many species 

 float freely about, but others are attached to plants etc. by long pedestals. 

 The pseudopodia are in the form of very delicate, tapering rays which extend 

 in all directions from the center, whence the name of sun animalcule. 



PROVINCE GREGARINIDA. 



Parasitic Animalcui.es. 



These peculiar minute forms of animal life are found parasitic within the 

 intestinal canals of earth worms, insects, crustaceans, etc. They are covered 

 with a membranous skin through which they absorb nutriment from the part- 

 ly digested food in which they float. Thus they have no need of pseudo- 

 podia, and consequently are without these appendages. They are, by some 

 naturalists, regarded as amoebae, and which have been placed under favor- 

 able conditions for obtaining food, and have thus become modified by their 

 surroundings. 



In propagation they undergo a kind of alternation of generation, similar 

 in some ways to that seen in much higher organisms. When about to mul- 

 tiply they become surrounded by a thick cyst, which may include one or two 

 gregarines. If there chance to be two, they loose their identity and merge 



