PROTOZOA. 49 



We pass naturally from the solitary Monosiga to the 

 colonial Codosiga pulcherrimus Jas.-Clk. PI. 51, fig. 7, is 

 the single, comparatively young zoon which has broken 

 away from its colonial home and is a free-moving animal. 

 During its young life it swims at times rapidly with its 

 basal end extending forward, and the flagellum following 

 behind "and vibrating in rapid undulatory and gyratory 

 curves as if it were the screw propeller of some sub- 

 aqueous vessel." 1 Finding a favorable spot whereon to 

 settle, the Codosiga secretes a stem and becomes a fixed 

 animal (fig. 8). The body is surmounted above by the 

 high collar. The dotted lines in the drawing indicate the 

 degree of the lateral vibratile expansion of the collar. 

 From the middle of the cone of the body extends the 

 flagellum. Figs. 9-16 of PI. 51 illustrate the process of 

 reproduction by fission, the stem not being drawn. At 

 first the collar bulged as seen in fig. 9. Soon after this 

 the flagellum grew shorter and finally disappeared, while 

 a narrow furrow was seen in the anterior part of the body 

 (fig. 10). This furrow extended downward, while the 

 collar became more cone-like (fig. n). Soon after this 

 the collar began to expand and the body was divided 

 about half way to its base. At each free rounded end a 

 flagellum began to be developed which kept up a trembling 

 motion (fig. 12). The body divided mostly to the base 

 and the collar broadened (fig. 13). The process of 

 division next extended into the collar (fig. 14) and con- 

 tinued, the collar growing broader and longer (fig. 15) 

 until finally the self-division of the collar and body was 

 complete and extended downward into the pedicel (fig. 

 1 6). A colony of two is often found; sometimes these 

 increase to five (fig. 17), and occasionally as many as 

 eight are produced. Fig. 18 is a free-swimming colony, 

 Desmarella moniliformis S. K. The early stages of this 

 species have not been observed, and therefore in the 



1 James-Clark, Mem. Boston Soc. Nat. Hist., I, 1866, p. 315. 



