68 SEX 



mate, whom he carries about with him for 

 prolonged periods. In some of the acorn- 

 shells there are pigmy males. 



In the other divisions of the Animal 

 Kingdom there is little illustration of sex- 

 dimorphism. The male cuttlefish often has 

 a peculiarly modified " arm " which becomes 

 a receptacle for sperms and is liberated into 

 the mantle cavity of the female. In the 

 paper nautilus (see Fig. 17) we have one 

 of those exceptional cases where the female 

 has a positive character which is unrepre- 

 sented in the male : she has two of the 

 arms remarkably modified and a very beautiful 

 shell used as a cradle for the developing eggs. 

 The male is much smaller and shows no hint 

 of an external shell (see Fig. 18). 



Mr. Cunningham has directed attention to 

 some examples of sex-dimorphism among 

 worms. In some marine Polychaetes, e. g. 

 species of Nereis, both sexes become extra- 

 ordinarily changed at sexual maturity so 

 much changed that mature forms have been 

 placed in one genus and immature forms in 

 another. There may be an alteration in the 

 shape and size of the locomotor appendages, 

 in the form and number of the bristles, and 

 in the size of the eggs and tactile structures. 

 But the point is that these alterations are 

 different in the two sexes, being more pro- 

 nounced in the males. In the Japanese 

 Palolo worm the females are deep green 

 above and greenish-yellow below, while the 

 males are light greenish-yellow above and 

 pinkish-white below. 



We may conclude this survey with mention 



