AMONG SIMPLE SABCODE ORGANISMS. 389 



A much more important character available for the definition of 

 the higher groups had been shown by Carpenter to be derivable 

 from the structure of the shell. In some this is throughout per- 

 forated bj'' minute canals, in others provided only with the single 

 comparatively large terminal orifice, or occasionally with a termi- 

 nal group of orifices, or in rare cases with two large orifices, one 

 at each end of the main axis. The Thalamopliora thus admit of 

 a primary division into the Imperforata and the Perforata, as in- 

 sisted on by Carpenter. 



It may be questioned whether the presence of a test aff'ords a 

 character of sufficient importance to justify its being made the 

 basis of such higher groups ; whether, for example, Arcella should 

 be far separated from Amoeba on the grounds that in one case the 

 sarcode is naked, in the other enveloped by a test. 



To this character Carpenter assigns a very subordinate value. 

 We must not, however, lose sight of the fact that the formation of a 

 test, — of a true test, at least, as distinguished from a mere pellicle 

 which yields to such changes of form as the sarcode may undergo, 

 — brings with it modifications in some of the most striking cha- 

 racters of the naked protoplasm ; for not only are the pseudopodia, 

 even in the Perforata, necessarily limited by it to definite points 

 of the body, but it substitutes a definite outline for the indefinite 

 and constantly changing outline of such naked organisms. The 

 importance of this definiteness of outline is shown by the great 

 symmetry which is in almost every case presented by it, while the 

 shell itself often possesses an elaborate structure, as seen in the 

 hexagonal areolae between the inner and outer tables of the shell 

 in Arcella and the beautiful tessellated structure in Qimdrula and 

 JEuglypha. 



Hertwig and Lesser have discovered many new freshwater re- 

 presentatives of the Thalamophora, and have made known to us 

 many important facts in their structure and life-history. All the 

 freshwater Thalamophora hitherto met with are nfonothalamic 

 (figs. 4, 5, &c.), and, if we except Archer's genus Diaphoropodon, 

 are also imperforate. Their test is of a conical or elliptical shape, 

 and is for the most part of a firm and solid consistence, though 

 occasionally membranous and flexible. It is either a pure product 

 of excretion, or in addition to this it may become more or less in- 

 corporated with foreign bodies, such as minute fragments of silica 

 or shells of Diatomacea. When it consists of a pure excretion 

 from the protoplasm-body it may be either perfectly smooth and 



