DIMORPHISM AND POLYMORPHISM 



365 



begins to dififer when metamorphosis of tlie larva into the sexu- 

 ally mature animal takes place. The larva is of an elongated 

 oval shape, is furnished with two bands of cilia by means of 

 which it swims about, and possesses an alimentary canal with 

 a mouth and anus. The transformation into the definitive form 

 of the species begins with the loss of the posterior band of cilia, 



'TTt 



Fig. 24. — Boiiellia viridis. (After Huxley, from R. Hertwig's "Zoology.") On 

 the left hand the male is shown, considerably magnified; and on the right the 

 female, about natural size, d. Rudimentary intestine of the male; z'd. Sperm 

 duct; i, Intestine of the female. 



and from this point onwards the mode of development is different 

 in the two sexes. The females grow rapidly, develop a ' pro- 

 boscis ' at the anterior end, and the intestine increases in 

 length ; the males, on the other hand, become entirely covered 

 with cilia, and the mouth and anus, as well as the fore- and 

 hind-gut disappear, the mid-gut, filled with yoke-granules, alone 

 being retained. Notwithstanding their great diversity, the two 

 sexes are formed on the same plan ; the male, however, may be 

 said in general to remain stationary at a certain stage of organi- 



