36 THE PROTOZOA 



and the daughter- individual forms a new shell for itself. In many cases the 

 shell formed by the daughter is larger than that of the parent ; for instance, 

 in Centropyxis aculeata and other species, in which the young individuals 

 multiply by fission, and each time they do so, the new shell formed is larger 

 than the old one, until the full size of the adult individual is reached 

 (Schaudinn, 131), after which point the new shell formed after the process 

 of fission is of the same size in both the parent and the daughter- individual. 

 In such cases the shell is always a single chamber, and is described technically 

 as " monothalamous." 



In other cases, however, the organism does not multiply by fission when 

 it has outgrown its first shell, but forms a new shell of larger size which is in 

 continuity with its first shell ; the protoplasmic body now occupies both the 

 chambers of the shell formed in this way. With further growth more chambers 

 are formed, giving rise to a complex " polythalamous " shell composed of 

 many chambers all occupied by the protoplasmic body (p. 232, infra). For a 

 detailed study of the developmental mechanics of shell-formation, see 

 Rhumbler (35). 



2. Internal Skeletal Structures. In many cases in which the proto- 

 plasmic body is naked at the surface, or bears only an extremely 

 thin cuticle, a definite body-form may be maintained by means of 

 internal supporting fibrils or other similar structures (Koltzoff, 

 30, 31). In some cases such structures may be of temporary nature. 

 A beautiful example of this is seen in the delicate organic axes 

 formed in the pseudopodia of Heliozoa (Fig. 22), in the form of 

 slender needle-like rods secreted by the protoplasm to stiffen the 

 pseudopodia, and absorbed again when the pseudopodia are re- 

 tracted. In other cases, supporting structures of organic nature 

 may be permanent constituents of the protoplasmic body ; such are 

 the axial rods, or " axostyles," found in many flagellates, such as 

 Trichomonas (Fig. 5, ax.), Lophomonas (Fig. 45), etc., slender flexible 

 rods of organic substance which form a supporting axis for the body. 

 Previous to division the axostyle is absorbed, and new axostyles 

 are formed in the daughter-individuals. The axostyles are stated 

 to arise from a centrodesinose (p. 103, infra) formed in the process 

 of division of the blepharoplast (Dobell, 236) or of the centriole of 

 the nucleus (Hartmann and Chagas, 62) ; the centrodesmose per- 

 sists after division is complete, and its two halves become the 

 axostyles of the two daughter-individuals. In Trichomonas eberthi, 

 however, Martin and Robertson (348) find that the axostyles arise 

 after division quite independently of the centrodesmoses or other 

 nuclear structures. In Octomitus (Fig. 116) two axostyles are present. 



From supporting structures of organic nature, such as the 

 axostyles or the organic axes of the pseudopodia mentioned above, 

 it is not difficult to derive the more rigid and permanent elements 

 known as " spicules," in which the organic basis becomes indurated 

 by deposits of inorganic mineral substance. In some cases spicules 

 may perhaps consist entirely of mineral substance deposited 

 directly within the living substance without any organic basis. In 



