274 THE COURTSHIP OF ANIMALS 



but the foot also is of a vivid scarlet, and the Pecten 

 have numerous minute eyes. But the Cockles and 

 Mussels possess like attributes as to colour and sculpture, 

 yet they are blind. More to the point is the fact that 

 these animals do not mate after the fashion of higher 

 animals, but the males, where the sexes are distinct, 

 discharge immense quantities of spermatozoa into the 

 water, and these find their way to the ova of the female 

 through the action of the inhalent currents set up by the 

 animal for the purpose of drawing in fresh supplies of 

 water containing food and oxygen. There are no 

 '' secondary sexual characters," that is to say, that even 

 where the sexes are separate, and many, like the Oysters, 

 are hermaphrodite, they are externally indistinguishable. 

 Nevertheless, many, as has been already remarked, have 

 shells of great beauty. As, for example, the giant ^ridacna 

 and the strangely spinous valves of the " Thorny 

 Oysters " {Sfondylida), 



The fact that the Lamellibranch, or bivalve molluscs, 

 are far less numerous in point of species than the univalve 

 tribes is accounted for by the fact that in the first place 

 they are of necessity aquatic, and in the second their means 

 of locomotion is extremely limited. Some few species 

 swim spasmodically : some crawl : many are incapable of 

 movement when once the motile larva settles down and 

 the shell-bearing adult stage is attained. Such species 

 can extend their range only by means of larval wander- 

 ings. Enormous numbers, millions, of young have to 

 be produced and set adrift each year by every adult 

 in the community, and yet but a f'ew of each brood can 

 ever attain to maturity. Life, for such species, must be 

 a dull, monotonous business : the only opportunity for 

 excitement is that which is preliminary to being eaten. 



