PEOTOZOA. 77 



calls to mind the streaming of tlie protoplasm in certain vegetable 

 cells. A nucleus, wliicli may be various in form, sliows tliat tlie 

 body of tlie Infusoriau is equivalent to a cell. In some tliere are 

 several nuclei. A liigher degree of potentiality is expressed by tlie 

 differentiation of various histological structures in the cortical layer. 

 But this does not affect the character of the Infusoria as unicellular 

 organisms, so long as they have only one nucleus, if we may suppose 

 that the cell is no longer in its indifferent condition. In many forms 

 there is a small nuclear structure, the nucleolus, in addition to the 

 nucleus. The Infusoria are divided into the Suctoria (Acuiita) 

 and the Ciliata. The former have definitely arranged fine pro- 

 cesses, capable of a small amount of movement, which serve in 

 the ingestion of food. The Ciliata are distinguished by an invest- 

 ment of cilia ; and subdivisions are formed according to the way in 

 which these cilia are distributed over the body. 



Bibliography. 



Rhizopoda : AuEEBiCH, C, Zeitschr. f. wiss. Zool. Bd. VII.— Dujaedis iii Ann. sc. I. III. IV.— 

 ScHULTzE, M., Ueber den Organismus der Polythalamien. Leipzig, 1851. — CAHPEifTEE, W., 

 Researches on the Foraminifera. Phil. Tr. 1856, 1859. — The same, Introduction to the study of 

 the Foraminifera. London, 1862. (R. S.) — Huxlet, Th. H., On Thalassicolla. Ann. nat. hist. 

 1851. — MiJLLEE, J., Abhandl. der Berliner Acad. 1858. — Hackel, E., Die Radioliirieu. Eine 

 Monographic. BerUn, 1862. — ScHULzE,F.E.,Rhizopodenstudien. Arch. f. mikr. anat. Bd. X — XII. 

 — Heetwio, R., Arch, f . mikr. anat. Bd. X. Suppl. — The same, Zm-. Histolog. der Radiolarien. 

 Leipzig, 1870. 



Gregarinse : Steix, Ueber die Natnr der Gregarinen. Ai-ch. f . Anat. u. Phyl. 18i8. — Kollikek, 

 Beitr. z. Kenntuiss niederer Thiere. Zeits. f. Zool. I. — LiEBEEKiiHif, Evolut. des Grigariues. 

 Acad. Roy. de Belgique. Mem. des Soc. ^trangferes. T. XXVI. Ed. van Beneden, Rech. sm- 

 I'evolut. des Gregarmes. Bull, de I'Acad. royale de Belgique. 2ine Ser. T. XXXI. Snr la Struct, 

 des Greg. Ibidem. T. XXXin. 



Infusoria : Ehrenbeeg, C. G., Die Infusionsthiere als vollkommene Organismen. Leipzig, 1838.— 

 DujAEDix, Hist. nat. des Infusoires. Paris, 1811. — Stein, Fh., Die Infusionsthiere auf ihre 

 Entwickelung uuter»ucht. Leipzig, 1851. — The same, Der Organismus der Infusionsthiere. I. II. 

 Leipzig, 1859-66. — Clapaeede, E., et Lachmann, Etudes sur les Infusoires et les Rhizopodes. 

 Geneve, 1858-61. — Engelmanx, Th. W., Zur Naturgeschichte der Infusionsthiere. Leipzig, 

 Zeitsclu'. f. Zool. XI. — M(jrphol. Jahrb. Bd. I. — Hackei, Z. Morphol. d. Infusor. Jen. Zeit- 

 sclu-ift VU. — BStschli, Archiv. f. mikr. Anat. IX. — Zeitschr, f. w. Zool. XXVIII. — Hekiwig, 

 R., Ueber Podophrya gemmipora. Morph. Jahrb. I. 



§ 61. 



As the body of the lowest organisms is formed of contractile 

 protoplasm, which changes continually in form, there is no definite 

 boundary to the body, nor any kind of differentiated integument. 

 We see the bodies of most of those Protista, which are not provided 

 with an envelope, alter in contour, just like the indifferent cells of 

 higher organisms ; processes of the protoplasm are extended from 

 different points at different times, and the rest of the body flows 

 after them. Thus, as the body moves, there is always a change in 

 its surface, so that a particle of its substance, which at one moment 

 is found in the interior, may at another moment enter into the 

 formation of a process. The processes, Pseudopodia, have some- 



