CRUSTACEA AND MOLLUSCS 245 



meet the requirement ; and in the second place, it is 

 clear that here the smallest variations had selective 

 value, because as they passed on to the land every 

 advance in the ramification of the blood-vessels in the 

 lining of the cavity meant a richer supply of oxygen, 

 and secured a longer life. 



There are plenty of examples of organs that had at 

 first a certain function, and were enabled from their 

 constitution to take up a secondary one. If a change 

 took place in the habits of the animal, this second 

 function might become the principal one ; in fact, the 

 first might gradually disappear, and the organ would 

 be correspondingly modified. We have a process of 

 this kind in the limbs of the craw-fish. It has feet on 

 what is called its tail, as well as the five pairs of legs, 

 of which the first bear the claws. The animal's tail 

 is really its abdomen, and must not be compared to the 

 tail of the vertebrate ; the alimentary canal runs through 

 it. The legs on this abdomen were at first clearly for 

 swimming, but were also used by the female for 

 carrying the eggs. The second function became more 

 important when the animal took to crawling as its chief 

 method of locomotion, and so the abdominal legs 

 became smaller and smaller by selection, and are now 

 only useful for carrying the eggs ; even in the male 

 they are used in connection with reproduction. 



Other legs of the craw-fish have undergone an even 

 greater transformation. All the limbs of the Crustacea 

 were originally legs, even the feelers, or antennae, that 

 act as organs of smell or touch, and the masticating 

 apparatus. Most of the masticating organs look very 



