PHYU'M PROTOZOA 



75 



which the}' are finally liberated. In Voho.r (Fig. 56), certain zooids, 

 called partlwnogonidia (A, «), have specially assigned to them 

 the function of asexual reproduction: they divide by a process 

 resembling the segmentation of the egg in the higher animals 

 (D^-D*), 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. 5ti. — Volvox globator. A, entire colony, enclosing several daughter-colonies; 

 B, the s.anie during sexual maturity ; C, four zooids in optical section ; Di — D5, develop- 

 ment of partlienogonidium ; E, ripe spermary ; F, sperm ; G, ovary containing ovum and 

 spenns ; H, oosperm ; a, parthenogonidia ; Ji. flagellurn ; ov. ovum ; O'-.v- ovaries ; pf/. pigment 

 spot ; 8^/. spermaries. (From Parker's Biology, after Cohn and Kirehner.) 



(Fig. 57, E^) and undergo complete fusion (E^ — E*) : the result of 

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

 walled sac, the zygote (E^), 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^ — F^). In Pandorina (Fig. 55) 

 the cells of the colony escape from the common gelatinous envelojje 

 (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). 



