IN THE ANIMAL KINGDOM. 77 



mation, or the development of external buds, in the Rizopods 

 and Sponges. In the latter case there is a tendency though 

 not so general as in the vegetable kingdom for the 

 gemmae to remain in adhesion with the basis from which 

 they have been budded off, so as to give rise to composite 

 multicellular organisms, whose configuration depends on 

 corresponding variations in the mode of gemmation. 



Of these aggregated Protozoan structures, the Foramini- 

 fera, and especially the Sponges, are the forms that attain 

 the largest dimensions. 



Indications, however, are not wanting of the occurrence 

 of sexual reproduction, though as yet they have been recog- 

 nized only in a few isolated cases. In Tethya, an animal 

 of this division, ova and spermatozoa have been detected by 

 Mr. Huxley.* They do not appear to be formed in special 

 organs, but occur in mixed masses within the spicular invest- 

 ment of the common organism. The other cases are those 

 observed by Balbiani among the Infusoria, as described in a 

 late communication to the Academy of Sciences of Paris. 

 This author has met with indications of the process in six 

 or seven species representing different groups, but confines 

 himself in this paper to a description of it as it occurs in 

 Paramcecium Bursaria, the species in which he has been 

 able to trace it most completely.')' For several generations 

 the Paramcecia multiply by spontaneous fission, each of 

 the two new individuals obtaining half the primitive nucleus, 

 but under the influence of conditions, of which we are still 

 ignorant, the species propagates itself by sexual concourse. 

 When the period for this arrives, the individuals are found 

 coupled together in pairs, adherent laterally, and, as it were, 

 locked together, with the similar extremities turned in the 



* Annals of Nat. Hist., 2d Ser., VII., 370. Also by Lieberkulm in 

 Spongilla, Op. Cit., XVII., 412. 



f Annals of Natural History, 3d Ser., I., 435. (Comptes Eendus, 

 March 27, 1858, p. 628). Greene Manual of Protozoa, p. 72. 



