30 LOWER INVERTEBRATES. 



viduals, and the part without contractile vacuole and mouth soonacquires them. Each 

 resulting infusorian, after a longer or shorter time, again divides. Ehrenberg isolated 

 examples of different species, and after ascertaining how long time was required for 

 division computed the enormous numbers produced by this process alone. In some 

 genera, by no means a limited number, the division occurs in the opposite direction, or 

 longitudmally ; it may not infrequently be seen in VorticeUa or J^stylis, so that the 

 large colonies of the latter have thus increased from a single zooid. In certain forms, 

 at least, the oral aperture, present before division, is lost, two new ones being formed. 

 Binary division in a limited number of species, for example Stentor, is oblique. 



An instance of multiplication by gemmation is 

 afforded by Semiophrya gemmipara (Fig. 23). The 

 buds appear on the anterior border of the parent 

 animal, the nucleus branches, sending out divertic- 

 ula into the buds. This phenomenon probably 

 takes place in other species which bud externally. 

 Nbctiluca miliaris also increases by this process : 



Fig. 23. — Hemwphrya, gemmiparay wim •' ^ 



eighteiongategemmuies, magnified 200 the nucleus first disappears, then the protoplasm 



divides, first into two, then four, and so on. These 

 masses are at length protruded upon the surface, the flagella are developed, and finally 

 they are liberated as free-swimming germs. 



Sporular multiplication, especially among the Flagellata, has been frequently ob- 

 served. The careful and patient researches by Messrs. Dallinger and Drysdale have 

 done much to acquaint us with the phenomena attending this process. In case of the 

 flagellate monads they found each sj)ecies to pass through several stages of develop- 

 ment in their life-history, viz. : the flagellate or mature form, the amoeboid, the encysted, 

 and the sporular condition ; the last appearing upon the breaking up of the contents of 

 the cyst. In a flagellate obtained in an infusion of cod-fish it was found that many of 

 these organisms all at once appeared to pour out a delicate sarcode, which exhibited 

 amoeboid movements. Two of these amoeboid masses would unite, after which the 

 sarcode became spherical, and at length developed the true cystic wall. Upon the 

 rupture of the cyst, there escaped multitudes of microspores, not large enough to be 

 individually defined by a magnifying power of fifteen thousand diameters. They were 

 continuously watched until they developed into the initial forms. Similar phenomena 

 have been recorded as taking place in the development of the higher Infusoria. 



Sexual, or genetic reproduction, in the sense of the union of two distinct and differ- 

 ent elements, has not been proven to occur. As stated above, individuals entirely 

 indistinguishable unite before sporular sub-division. It is also well known that indi- 

 viduals unite transiently, and then separate to each continue multiplication by binary 

 division. That the repeated sub-divisions so exhaust the stock that it must necessarily 

 be revitalized by the conjugation of separate individuals is generally held. It is also 

 recognized that the nucleus and nucleolus play significant parts in the rejuvenesence 

 due to the zygosis, or conjugations. Concerning the interpretation of the facts there 

 is much difference of oj)inion. A notice of the several views cannot be introduced here. 



O. F. MllUer, in 1786, first attempted to classify the Infusoria. Eleven of his seven- 

 teen genera are still recognized in the infusorial class. The number of species included 

 was about two hundred. Ehrenberg's more elaborate system, published in 1835, der 

 scribed three hundred and fifty species (after deducting the rotifers and plants), and 

 separated them into sixteen families and eighty genera. Von Siebold, in 1845, divided 



