4 LOWER JNVEUTEBllATES. 



from cyst to swarm-spores, and e being the first stage of reversion from swarm-spores 

 to the mature form. 



The swarm-spores, to which the name plastidules has been given, are masses of 

 apparently structureless protoplasm, manifesting life in its simplest conceiA'able form. 



Class II. — KHIZOPODA. 



No definite boundary can be drawn between the Monera and the Rhizopoda, and it 

 is doubtful if the simple Protomyxa just described as a typical Moner, does not justly 

 deserve to rank in this class. In a general way it may be said that the Rhizopods are 

 distinguished from the Moners by having a more or less well-defined outer layer of sar- 

 code and a nucleus, although the latter is not always to be observed. 



The Rhizopoda have been divided by Dr. William B. Car])enter into three groups, 

 distinguished by the character of their sarcode or pseudopodia ; the Lobosa, in which 

 the pseudopodia are lobose or finger-like, as shown in the illustration of Amoeba i~iro- 

 teus, Fig. 2 ; the Radiolaria, in which the sarcode extends outward in rays more or 

 less constant in form and position, as in jictinophrijs, Actinospluerium or Clath- 

 rtdina, Fig. 10, among fresh- water forms, and Motalia, Fig. 15, among the marine 

 forms ; the Reticularia, in which the sarcode extends in irregular, soft anastomosing 

 branches, which coalesce whenever they come together, as well illustrated in Gromia, 

 Fig. 11. Dr. Carpenter groups all the Rhizopods under these three heads, as fol- 

 lows : — 



This arrangement is not founded upon any physiological or morphological distinc- 

 tions, and it can only be regarded as provisional. The different groups merge into one 

 another so that the character of the pseudopodia alone is a very unsatisfactory guide, 

 especially in distinguishing between the lobose and the reticularian forms. The 

 Radiolaria are the most complex in structure of all Rhizopods ; the marine forms pro- 

 duce silicious skeletons of great variety and beauty. The Reticularia include the 

 marine Rhizopods with calcareous shells, often quite complex in structure ; some of 

 them grow to a comparatively large size. The immense beds of chalk of the Old World 

 are largely composed of the shells of Foraminifera, and the Eozoon canadense, claimed 

 by some to be the oldest form of animal life known to the geologist, if really an ani- 

 mal, also belongs to this group. 



Our fresh-water Rhizopods have been treated very fully by Prof. Joseph Leidy in 

 his "Fresh-water Rhizopods of North America." He has di\ided them into the Proto- 

 plasta, from pirotos, first and julasso, a form or mould ; and the Jleliozoa, from helios, 

 the sun, and zoon, animal. The Protoplasta are divided into the Protoplasta lobosa, 

 which corresponds to the Lobosa of Carpenter, and Protoplasta filosa, which are 

 included in the Reticularia of Carpenter. The Heliozoa, which live in fresh water, 

 are closely alUed to the mai'ine Radiolaria, but their precise relations are not yet 

 understood. 



