AMONG SIMPLE SAECODE ORGANISMS. 391 



Another important systematic character is derived from the 

 number of terminal or oral orifices in the test, though this, in 

 consequence of the great inequality of the two groups based upon 

 it, loses much of the practical value which it would otherwise 

 possess in classification. In almost every case there is but a single 

 such orifice (figs. 4, 5, 6, &c.). In some rare cases, however, there 

 are two, one situated at each end of the main axis (fig. 8). It is 

 these conditions which Hertwig and Lesser designate respecbively 

 by the names of Monostomatous and Amphistomatous. 



Among the freshwater Thalamophora with blunt pseudopodia 

 (Lobosa) Hertwig and Lesser* have made some interesting obser- 

 vations on the long-known and widely distributed Arcella vulgaris, 

 one of the largest and best fitted for observation of the freshwater 

 BJiiz.opoda. They have corrected the descriptions of the structure 

 of its shell given by previous observers, and have brought together 

 its essential characters in a more exact generic diagnosis than had 

 been hitherto attempted. 



Its shell, which is a pure excretion from the contained proto- 

 plasm, has its main axis very short in proportion to the lateral 

 axes ; and the elongate form so characteristic of the freshwater Tha- 

 lamophora becomes thus shield-shaped with the orifice for the 

 pseudopodia in the centre of the flat side, which, during locomotion, 

 is turned towards the supporting surface. Its walls, as now shown 

 by Hertwig and Lesser, are composed of two parallel plates, an 

 outer and an inner, which are united to one another by an interven- 

 ing structure with hexagonal chambers like those of a honeycomb. 



Besides containing contractile vesicles the protoplasm is very 

 exceptional in containing numerous nuclei. The body does not 

 entirely fiU the shell, but forms a disk-shaped mass of protoplasm 

 lying on the lower wall and having the contractile vesicles and 

 nuclei immersed in its peripheral parts. It is connected with the 

 upper wall of the shell by filiform processes, which in young spe- 

 cimens are richly developed, repeatedly branch and anastomose, 

 and form a sort of pseudopodial net over which the protoplasm- 

 granules travel to and fro. 



An encysting process, apparently connected with reproduction, 

 has been observed by them. In this the protoplasm becomes 

 surrounded by a globular cyst which lies within the shell close to 

 the orifice. The coarsely granular and opaque condition of the 

 protoplasm rendered it impossible for them to discover any thing 

 * Arch. f. mikr. Anat. Band x. Suppl.-Heft. 



