CHAPTER YII 



MOLLUSCA, CMTOPODA, AND LOWER SUB-KINGDOMS 

 MOLLUSCA 



It was Darwin's opinion that in the sub-kingdom, Mollusca, 

 secondary sexual characters, such as he considered in The 

 Descent of Man, never occurred. He mentions the special 

 development in male Cephalopoda (cuttle-fishes) of one of the 

 arms, which acts as the carrier of the spermatophores, but 

 thinks that this may be classed as a primary rather than as 

 a secondary sexual character. This conclusion seems to me 

 unjustified. It is evident from hundreds of cases that nothing 

 beyond the ovaries and testes, not even the ducts of these 

 organs, are really primary in the proper sense of the word. 

 I take it that the primary or essential organs are those 

 which are necessarily or invariably required for the function 

 of reproduction, and in Ccelenterata and Sponges reproduction 

 is effected by ova and sperms without the aid of any other 

 specialised organ of the body. It is true that in Mammals 

 and other classes of the higher animals it is customary to 

 consider such characters as horns or plumage as secondary 

 sexual characters, and not to include the actual organs of 

 copulation. But nevertheless the organs of copulation are 

 truly secondary, and are to be explained on the same prin- 

 ciples as sexual differences of a more remote kind. Sexual 



