86 ORGANIZATION OF THE INFUSORIA. 



millions might be similarly developed within the space of a single month. 

 Substantial evidence of the prodigious numbers that may be produced 

 by this simple reproductive mode is afforded by the various species be- 

 longing to both the Ciliate and Flagellate sections of Infusoria that 

 are characterized by their sedentary and colonial mode of growth, each 

 colony-stock under such conditions representing the sum total of the 

 repeated binary subdivision of a primary single unicellular zooid. Ophry- 

 dium versatile constructs in such a manner aggregated masses, derived 

 primarily from a single animalcule, that vary in size from that of a walnut 

 to a child's head, while Epistylis grandis similarly produces on submerged 

 plants, or the walls of an aquarium, what appear to the unassisted vision 

 as homogeneous slime-like masses of many feet in extent. Correspond- 

 ingly derived colonial aggregations, though on a somewhat smaller scale, 

 are encountered among the several Flagellate genera Dendromonas, Spongo- 

 monas, Phalansterium, and A nthophysa, while in the group of the Spongida, 

 that must be regarded as peculiarly modified colonial aggregations of 

 such Flagellate types as Codosiga and Phalansterium, a composite struc- 

 ture is produced by a chiefly though not entirely similar process, which 

 in many cases far exceeds in bulk the extensive colonies of the several 

 Ciliate forms first quoted. 



The phenomena of multiplication by binary division or fission are found 

 to manifest several characteristic modifications. With the majority of species 

 such division takes a cross-wise or transverse direction, a groove or con- 

 striction making its appearance towards the centre of the body, and 

 becoming gradually deeper and deeper, until at length the anterior and 

 posterior halves become entirely separated from one another, each swimming 

 off as an independent animalcule, and such organs as might be wanting 

 to either separated moiety becoming rapidly developed. In a somewhat 

 smaller, but still pretty considerable assemblage of types, the process 

 of fission is manifested in a precisely opposite or longitudinal direction. 

 Such a plan of multiplication is more essentially characteristic of the large 

 group of the Vorticellidae, and those representatives of the Flagellata, in 

 which, as with many of the former, a compound tree-like colony-stock is 

 built up. Illustrations among these latter are especially afforded by such 

 genera as Codosiga, Dendromonas, and Rhipidodendron. The compound 

 zoothecium of Dinobryon, while at first sight apparently constructed by 

 means of a similar reproductive formula, is the product of transverse 

 fission, each anteriorly separated zooid utilizing the wall of the lorica it 

 quits as the fulcrum of attachment of its own independently constructed 

 domicile. The most remarkable modification of longitudinal fission is 

 perhaps furnished by the collared Flagellate genus Desmarella, and in 

 which the variable number of zooids developed by this process remain 

 laterally attached in such a manner as to form a more or less elongate 

 necklace-like series. In the genus Vorticella and its allies, as hereafter 

 recorded, the process of fission is always preceded by the closing up and 



