Protozoa.- 



-RADIATA.- 



-Infusoria. 



403 



few species, and those are all natives of fresh water. 

 The animals have their tentacles disposed in a crescentic 

 or horse-shoe series round the disc surrounding the 

 mouth. The order is well represented by the " Bell- 

 flower Animals" of some of the earlier writers on the 

 microscope, the genus Plumatdla. Another genus, 

 CrUiatdla, is remarkable from the only species be- 



longing to it possessing, when adult, a locomotive 

 faculty. This creature is a native of the fresh waters 

 of Great Britain, and is found floating on the surface. 

 Though it does possess motion, its progression is very 

 slow, seldom exceeding an inch in twelve or twenty- 

 four hours. This beautiful species forms a fine object 

 for microscopic inspection. 



Class IV.— PROTOZOA. 



The Class Protozoa contains an immense number of 

 creatures, very minute in size, and very low in organi- 

 zation. They all belong to two large orders — 1. In- 

 fusoria; 2. Rldzopoda. 



Order I. — INFUSORIA (Infusory Animalcules). 



The animals belonging to this order of animals are 

 for the most part extremely minute, and have been 

 made known to us by the aid of the microscope. They 

 inhabit stagnant water, fresh or salt, in which plants 

 are growing, or in which an abundance of decayed 

 vegetable or animal matter is contained. Most of 

 them are invisible to the naked eye, and species have 

 been described from l-1500th to l-2000th of a line 

 in length. So exceedingly minute, indeed, are they, 

 that it is calculated a moderate-sized drop of water 

 may contain five hundred millions of these minims of 

 creation ! They are of very simple organization, as 

 they are furnished with neither vessels nor nerves, are 

 not symmetrical, have not distinct sexes, have no 

 visible eggs, and are without determined or permanent 

 digestive cavity. The chief organs of which they 

 are composed appear to be internal spherical cavities, 

 which frequently contain foreign particles supposed to 

 serve as food, and which are derived from the sur- 

 rounding water, entering into the substance of the body, 

 and moving through it by the general contraction of 

 the mass, and being expelled through the opposite 

 side. Some have no visible locomotive organs, others 

 have ; and these are either cilia, or changeable processes, 

 as they are called— expansions of the substance of the 

 body. The substance itself which forms the body is 

 called Sarcode, and in perhaps the greater number is 

 a glutinous, homogeneous, or slightly granular diapha- 

 nous mass. In some Infusoria red specks are visible, 

 which resemble eyes, and by some zoologists are really 

 considered as such ; though, from a want of all traces 

 of a nervous system, and possessing no distinguishable 

 cornea or lens, others deny them to be true eyes, and 

 compare them to the red specks often seen in Algje. 

 The food of the Infusoria consists partly of vegetables, 

 and partly of animal decomposing matter, and the}' 

 not unfrequently prey upon each other. They are the 

 prey, again, of other aquatic animals; and as they 

 occur in immense quantities in the water of the ocean, 

 as well as in that of our fresh-water pools and slow 

 running streams, the}- contribute much to the nourish- 

 ment of animals of a higher order which are useful to 

 man. This has been particularly observed in the ocean. 



in high latitudes, where vegetable life ceases to be 

 represented, but where animal life is still in abundance. 

 Infusoria are there found to exist in inconceivable 

 numbers, and form the principal food of the fishes 

 inhabiting these regions. They frequently occur in 

 such immense numbers as to coloiu- large tracts of 

 water whh very remarkable hues. Some species im- 

 part a blood-red tint to the water, and Ehrenberg has 

 thrown out the conjecture that these little creatures 

 were the agents employed in the miracle worked by 

 Moses in turning the waters of Egypt into blood ! 

 Others give a blue colour, wdiile a third set tinge the 

 whole surface with green. At sea, though singly in- 

 visible, they produce one kind of the phosphorescent 

 light so often observed in the ocean. Formerly, before 

 their history was well understood, they were errone- 

 ously considered to derive their being, directly and 

 spontaneously, from the decomposition of animal or 

 vegetable matter in water, and were, from their sudden 

 appearance in such situations, and in such immense 

 numbers, held to prove the truth of what is called 

 " spontaneous generation." Infusoria can resist a de- 

 gree of cold equal to 8° of Fahrenheit, and a degree of 

 heat equal to 260° Fahrenheit. According to Ehren- 

 berg, who has studied these creatures with great care, 

 they are divided into two large groups, which he de- 

 nominates PoJygastrica and Rotifera. 



This latter group, however, has now been proved to 

 be of a much higher organization than the former, and 

 has accordingly been removed from the class Infusoria 

 into a separate class by itself, allied to the Crustacea. 



The Pohjgastrica have also been proved to contain 

 whole families and sections which are not true lufusoria. 

 Thus the Diatomaccce and Desmidiacem have been 

 proved to be forms of Ahjce. The Vibrionina are now 

 included amongst the OsciHatorew, a family of com- 

 pound Alr/a;; whilst the Volvocian !\nd Monad ina are 

 ascertained to be other forms of that large order of 

 plants. On the other hand, many others are appa- 

 rently only the larva state of Annelides. The genus 

 Paramecium, for example, has been ascertained to be 

 the embryo state of Planaria, and the genus Cercaria 

 to be the undeveloped young of an intestinal worm ; 

 and lastly, two whole families containing the genera 

 Amiba, Arcellina, &c., have been removed to form part 

 of the following order, Rhizopoda. 



The greater portion of the fossil Infusoria described 

 by naturahsts as so abundant in the marine and fresh- 

 water deposits, are in like manner removed from this 

 class and placed amongst the Diatomaccce. Though 



