OF THE RUIZOPODA IN GENERAL :—LOBOSA. 



SI 



29. From the simple naked Amceba a transition is presented to those testaceous forms 

 which accord with it in general structure, by the genus Fseudochhimys, in which the dorsal 

 surface is protected by a buckler or cai-apace (somewhat resembling the shell (f a FateJJa), 

 whose substance, however, is not dense enough to prevent its changing its form with that of 

 the body. In the P. patella described by MM. Claparede and Lachmann (xxv, p. 443), which 

 is common in ponds in the neighbourhood of Berlin, from six to ten " contractile vesicles" are 

 seen along its margin, Imt only a single " nucleus" can be distinguished. 



30. Of the testaceous Amcebina there are two principal types ; the ArceJltB, which form a 

 membranous " test," possessing considerable firmness in itself; and the Difflugia, in which 

 firmness is imparted to the " test" by the adhesion of foreign particles (such as grains of 

 saud or fragments of shell) to its external surface. Both are inhabitants of fresh water, and 

 are pretty generally diffused. They have been divided into many species, according to 

 supposed differences of the most trivial character, which extended observa,tion proves to be 

 altogether valueless. 



31. The "test" oiArcdla is ordinarily of nearly hemispherical form, the circular plane which 

 closes it in being perforated by a large aperture, from which the pseudopodia are put forth 

 (PI. I, fig. 19) ; sometimes, however, it is deepened into the form of a vase or pitcher, wliilst 

 in other instances it is flattened or depressed. Its surface is usually marked with hexagonal 



. facets, which are due to the thinning of the membrane at those parts ; and sometimes it 

 presents a larger pattern formed by bands or depressions passing round it in various directions. 

 The animal . creeps with the mouth of the shell downwards ; its pseudopodia, which resqmble 

 those of Amceba, being extended between the flattened side of the shell and the surface over 

 which it moves. According to MM. Claparede and Lachmann (xxv, p. 444) ; the number of 

 " contractile vesicles" in ArccUa is very variable, and may increase with the bulk of the animal ; 

 they are always disposed around the periphery of the hemisphere. Although individuals are 

 sometimes observed to possess only a single " nucleus," yet this body is generally repeated 

 several times ; and these multiple nuclei may be seen to form a ring interior to the contractile 

 vesicles, which they may equal in number, as many as from ten to fifteen having been 

 observed. The Arcella may be observed to change its " test" many times during the course of 

 its life, not having any power of enlarging it. When the sarcode-body has become too 

 bulky for its envelope, it projects from the aperture of the test, from the cavity of which it 

 almost entirely withdraws itself ; and on the surface of this projecting portion a new test is 

 formed, which often differs from the old one in those characters which have been considered 

 to diS'erentiate species. At first, the new test is thin, transparent, and colourless ; but it 

 gradually becomes thicker, more opaque, and of a light yellowish-brown colour. The old and 

 the new tests are applied to one another, aperture to aperture (a condition which seems to 

 have been mistaken for an act of " conjugation") ; and the sarcode-body passes alternately 

 from one to the other, until at last, when the new test has acquired sufiicient consistence, 

 the Arcella entirely quits the old one, which very commonly splits under the violence that 

 seems involved in the final separation (xxv, pp. 44.5, 446). The "test" of Bijjlugia (fig. 20) 

 is usually more or less of a spherical or ovoidal shape, but is sometimes more elongated into 

 the form of a pitcher or a flask ; its aperture is generally somewhat notched. When 



