262 GENERAL HISTORY OF THE INFUSORIA. 



SUPPLEMEl^TARY FAMILIES OF PROTOZOA. 



A. — Gregarinida. Their General Characters, Structure, and Affini- 

 ties. — The Gregar'inida constitute one of the three groups into which several 

 eminent naturalists subdivide the Protozoa ; they therefore claim from us a 

 brief description. 



They are of the most simple structure ; indeed, some writers place them 

 below the Rhizopods in the animal series, because, unlike these, their simple 

 type undergoes no further elaboration or developmental complication. They 

 are parasites, living in the visceral cavities of other animals, and in their 

 simple structure are comparable to a cell, or to the o^iim of higher animals. 

 Thus they consist of a homogeneous albuminous -like matter, -with numerous 

 granules of coarser and finer character and fat-like globules, enclosed within 

 a membrane of more or less perfect structure, which in all essential points 

 I'epresents a cell-wall; besides, they have always one distinct central vesicular 

 body or space containing one or more granules, and evidently of the nature 

 of a nucleus. Of these parts, the general mass may be taken to resemble the 

 yelk-matter, and the nucleus the germinal vesicle of an ovum. 



The enclosing membrane is very yielding, and admits of great and constantly 

 fluctuating alterations of figure by the varying contractions and extensions 

 of the internal contractile mass ; but there is no such thing as the formation 

 of pseudopodes, as happens among Ehizopoda. It is entire, without orifice 

 either in the shape of a mouth or anus ; consequently no foreign particles are 

 ever seen in the interior. Moreover, the Grer/arinida contain no contractile 

 vesicle, and have never been found to undergo either fission or gemmation. 

 Their vital endowments are so shght, that their animahty is at fii'st sight 

 doubtful ; but, imlike vegetable organisms, their envelope contains no cellulose. 



The above brief account comprehends all that can be stated generally of the 

 organization of these simple creatures, which, if above the Amoebcea in the 

 possession of a more or less definite membrane, yet sink beneath them in not 

 possessing a contractile vesicle. 



Notwithstanding their simplicity of structure, they yet are truly animal 

 organisms, enjoying an independent existence, manifesting the phenomena of 

 motion, growth, nutrition, and reproduction, in the last of which they exhibit 

 a peculiar cycle of changes. 



Moreover, there are various notable difierences between the various Grega- 

 rinida known, with respect to size, figure, to the activity of their functions, 

 and to some minuter points of structm^e. Hence theii- division into genera 

 and species. 



In size they vary from foiu' or five lines (as in the genus Didymojplirys) to 

 a few thousandths of an inch. Of their figure, some are simply rounded or 

 oval sacs, as in Monocystis ; others constricted around the middle, e. g. Grega- 

 rinida. Again, the majority have a smooth, naked membrane, whilst others 

 are armed with a ring of uncini at one extremity, like many Hehninthidce. 



When two nuclei occur in a single animal, it probably betokens an act of 

 reproduction. The encysting process is exhibited among the Gregarinida, in 

 connexion, however, onl}^ with their reproductive processes, and has this pe- 

 culiarity, that it does not occur to a single individual, but to two together, 

 which become enclosed within the common cyst or capsule. In their progress 

 to this union the two Gregarince are seen first to approach, and then by mu- 

 tual pressiu^e to flatten, the opposed surfaces, so that the binate being acquires 

 a globular form. The substance to form the cyst is in the meantime thrown 

 out, of a soft gelatinous consistence, but gradually becomes condensed and 

 contracted into a membranous -looking capsule. 



