90S 



THE ANIMAL KINGDOM. 



constantly uninucleate ; Arcella and one Amoeba (A. binucleata) are typi- 

 cally binucleate ; some Amoebae, Pelomyxa^ Zonomyxa, invariably multi- 

 nucleate ; and in this case the number of nuclei may be constantly limited, 

 e.g. to not more than eight in Amoeba tertia, or it may be indefinite and 

 reach a high figure, e.g. some hundreds in A. quinta and Pelomyxa palus- 

 tris. But both uni- and bi-nucleate forms may become multinucleate, 

 e. g. Arcella, with 6, or even 40, Difflugia proteiformis with 40, and the 

 increase is doubtless connected, as in Foraminifera, with reproduction. 



Binary fission has been observed in some Nuda, and is probably 

 universal among them. Among Testacea, in Pamphagiis (Lecythium) 

 longitudinal fission takes place and includes the delicate test ; similarly 

 transverse fission in Diplophrys Archer i. But in those Testacea where the 

 test is rigid, a process of modified fission or gemmation occurs, observed 

 in all its details in Euglypha alveolata by Gruber. A number of concavo- 

 convex plates lay round the nucleus at the base of the test, which was 

 entirely filled by protoplasm : the latter slowly protruded from the aper- 

 ture of the test, and the loose plates passed by degrees up its sides out of 

 its aperture, and arranged themselves on the surface of the swelling mass 

 of protoplasm. As soon as this mass had attained the same size as the 

 individual from which it was derived, the nucleus of the latter became 

 greatly elongated, and its chromatin filaments arranged in parallel longi- 

 tudinal lines ; it then underwent fission, and one part passed into the new 

 individual. When the transfer was completed, lively currents in the pro- 

 toplasm were set up in and between the two Euglyphae. On their 

 cessation the protoplasm was somewhat retracted from the walls of the 

 tests, pseudopodia were extruded at the line of junction between their 

 apertures, an^l then came separation. A similar process was observed, 

 incompletely however, in Cyphoderia. From the fact that tests are often 

 seen in apposition by their mouths, that loose plates have been seen 

 within empty tests, e. g. in Quadrula, Gruber believes that the process, as 

 witnessed in Euglypha, is general among Testacea, the material of the 

 new test, whether plates, chjtinous prisms or foreign bodies, being furnished 

 by the parent. Individuals of Arcella are frequently seen in apposition, 

 the shell of one being colourless, or nearly so, indicating its newness. But 

 it is possible that in Arcella, and certain that in Platoum stercoreum, the 

 new test covering the protoplasmic protrusion is formed by the protrusion 

 itself. Whether conjugation ever really occurs is doubtful ; but indi- 

 viduals of Arcella and of Difflugia globtilosa, with tests evidently old, have 

 been found united aperture to aperture. And in the first-named the pro- 

 duction by gemmation (?) within the test of amoeboid young, with nucleus 

 and contractile vacuole, and their subsequent escape, has been seen to 

 follow this union (Butschli). Similar amoeboid bodies, produced by gem- 

 mation or repeated fission in Arcella, have been traced until they acquired 



