OF THE PROTOZOA. 



277 



their alliance to this family somewhat doubtful. They are met with in chalk, 

 the only secondary stratum, and here in the substance of flints ; but they 

 also occur in strata of later formation. Their presence in flints renders it, 

 indeed, sujDposable that their silicious constitution is an ulterior result of the 

 infiltration of silex in a state of solution into the texture of theii' previously 

 membranous envelope. They are found in company with fossil Fyxidicula 

 and Xanthidia. Ehrenberg described two fossil species under the name of 

 Ceratium pyro^horum and C. Delitiense. 



CILIATA. 



Grotjp B. — Stomatoda. 



(Illustrated by Plates XXIY.-XXXI.) 



The animalcules whose general history we have noAv to write are, as before 

 mentioned, comprehended for the most part in the families Dinohryina, 

 Vort'wellina, Ophrydina, EncJielia, Colejmia, Trachelina, Opliryocercina, Aspi- 

 discina, Koljjodea, OxytricJiina, and Euplota, as instituted by Ehrenberg, with 

 the removal of the OpaUncea from the Trachelina, and of the Acinetina and 

 Actinopliryina from the Enclielia. 



The descriptions of the beings composing these several families, as furnished 

 by Ehrenberg, are so tinged by his peculiar views of organization as to mar 

 their utility ; and therefore, for precision and accuracy of detail, we have to 

 rely in great measure on the observations made within the last few years, 

 chiefly by German naturalists. Notwithstanding the jDersevering industry 

 •vvith which these scientific men have piu'sued their inquiries, many genera 

 yet remain almost imknown, or little understood, in respect to their structm-e, 

 whether internal or external. 



The Ciliated Stomatoda, or as we shall more briefly style them the Cihata, 

 are microscopical animals having a defijiite limiting membrane or external 

 tunic covered more or less completely with vibratile cilia, by which they 

 swim ; and when it is indurated, as not unfrequently happens, it is further 

 furnished with bristles or other tegiunentary appendages, by which they 

 are capable of crawling or leaping. They aU possess a more or less di- 

 stinct mouth, which opens into an oesophagus or gullet, continued to a vari- 

 able extent into the interior as a digestive or alimentary tube, but ending 

 abruptly by an open extremity. In many genera a discharging orifice or 

 anus is perceptible ; and in all there are a nucleus and one or more contractile 

 vesicles. They propagate by self-di\^sion, by gemmation, and by internal 

 germs or embryos, with a greater or less degree of metamorphosis, and they 

 undergo the encysting process : the act of gemmation appears limited to a 

 few genera ; but self- fission and embryonic development may be predicated as 

 general phenomena. 



Dimensions. — In dimensions aU the Cihata are microscopical ; for if some, 

 such as Spirostomum, Stentor, Opercidaria, Zoothamnium, Vayinicola, and 

 other genera of Volvocina and Ophrydina are visible to the naked eye as 

 minute specks or globules, they are far beyond its ken for any purposes of 

 investigation, and are therefore essentially objects for the microscope. Yet 

 amid these hosts of equally microscopic beings, the range in point of size is 

 actually as great as that between the dog and the elephant among animals cogv 

 nizantto our ordinaiy observation. Even among members of the same genus, 

 and, indeed, of the same species, their dimensions may vaiy within limits 

 extremely wide. To quote a few examples : Spirostomion ambigimm (Ehr.) 

 has a length of ^th of an inch ; the branching polyparies in EpistyUs and 

 Opercidaria reach ith in height, those of Zoothamnium 1th, whilst many 



