70 



ZOOLOGY 



SECT. 



which they are finally liberated. In Volvox (Fig. 50), certain 

 zooids, called partlienogonidia (A, ), have specially assigned to them 

 the function of asexual reproduction : they divide by a process 

 resembling the segmentation of the higher animals (D^D 5 ), and 

 form daughter-colonies which become detached and swim freely 

 in the interior of the mother-colony. 



A very interesting series of stages in sexual reproduction is 

 found in this group. In Heteromita two individuals come together 



FIG. 50. Volvox globator. A, entire colony, enclosing several daughter-colonies ; 

 B, the same during sexual maturity ; C, four zooids in optical section ; D 1 D5, develop- 

 ment of parthenogonidium ; B, ripe spermary ; F, sperm; G, ovary containing ovum and 

 sperms; H, oosperm ; a, parthenogonidia ; fl. flagelluru ; or. ovum ; or//, ovaries ; pg. pigment 

 spot ; spy. spermaries. (From Parker's Biology, after Cohn and Kirchner.) 



(Fig. 51, E 1 ) and undergo complete fusion (E 2 E 4 ) : the result of 

 this conjugation of the two gametes or conjugating cells is a thin- 

 walled sac, the zygotc (E 5 ), the protoplasm of which divides by 

 multiple fission into very minute spores. These, when first 

 liberated by the rupture of the zygote (E), are mere granules, 

 but soon the ventral or trailing flagellum is developed, and after- 

 wards the anterior flagellum (F 1 F 4 ). In Pandorina (Fig. 49) 

 the cells of the colony escape from the common gelatinous envelope 

 (C) and conjugate in pairs (D, E), forming a zygote (F, G), which, 

 after a period of rest (H), divides and forms a new colony (K). 



