ASEXUAL REPRODUCTION. 



181 



merits. A physiological link, however, between worms of only one segment 

 and those with many, is found in the asexual chains which some of the former 

 occasionally develop. Thus the little turbellarian Microstomwm li?ieare may 

 bud off a temporary chain of sixteen individual links. The 

 budding begins at the posterior end, ..nd what is partly sepa- 

 rated off is a portion in excess of the normal size. The 

 second link grows till it attains the usual adult size, and as it 

 exceeds this form a third link. At the same t'. ne the original 

 individual may also be doing the same, and thus a chain of 

 four is formed. Two more buddings by each of the links 

 bring the asexual process to a climax, and then the individ- 

 uals separate from one another and become sexual in free- 

 dom. It is important to notice that the asexual reproduction 

 takes place in favorable nutritive conditions, and as each 

 individual exceeds its normal limit of growth. In some allied 

 planarians the asexual multiplication is effected not by bud- 

 ding but by division. Zacharias observed that when nutri- 

 tion was checked the vegetative increase ceased, and sexual 

 reproduction set in. Not quite parallel with the above, but 

 quite asexual, is the prolific multiplication characteristic of 

 the flukes and tapeworms. The common liver-fluke has often 



several asexual generations before it finds its final hosts in 

 the sheep, and is surpassed in this respect by some of its 

 relatives. The bladder worm, in passive ease, with a plethora 

 of nutrition, may form asexually many "heads," each of 

 which, inside a future host, grows out into the long series of 

 joints which compose the tapeworm. In their profuse asex- 

 ual multiplication these parasites are like parasitic fungi, but 

 unlike them in the retention of the sexual process to boot. 



In their asexual reproduction, the Polyzoa recall sponges, 

 for not only do they all multiply by budding, and that abundantly, but they form 

 peculiar winter-buds like sponge-gemmules, by which on the death of the parent 

 the continuity of life is nevertheless sustained. The winter-buds or statoblasts 

 may further resemble sponge-gemmules in elaborateness of ex ernal equipment, 

 a common characteristic of passive resting structures. 



Fig. 56. — Diagram- 

 matic representa- 

 tion of the forma- 

 tion of a chain of 

 individuals in the 

 Turbellarian worm 

 (Microstomttm lin- 

 care). — From Leu- 

 nis. 



Fig. 57. — A Sea-worm (Myrianida) which has budded off a chain of individuals. 



— After Milne-Edwards. 



