METAMORPHOSIS OF COKYSTES CASSIVELAUNUS. 463 



tliat the first trace of the auditory invagination is to be 

 found. It seems likely^ then^ that the spines may perform to 

 some extent the balancing and orienting function of the 

 auditory sac. 



Besides being of balancing and directive function^ the 

 spines probably also serve as a protection. It is^ of course, 

 hard to say what are the especial enemies of the zo^as in 

 nature. They are certainly preyed upon to some extent by 

 Medusee and Ctenophores, and also by each other, and in 

 these cases the spines can be of little value. They must, 

 however, serve as a defence against the attacks of small fish. 

 That this is so was shown by presenting the zoa3a to Gobius 

 ruthensparri, a fish about 4"5 cm. long, and with a width 

 of mouth of about 3"5 mm. In the first experiment a goby 

 seized and rejected the zoEea six times, each time failing to 

 swallow it. Finally the fish gave up the attempt, and the 

 zoaea soon recovered and swam away, being, however, 

 attacked and swallowed by a second goby after a number of 

 failures. Other experiments showed the same thing, though 

 the fish had no difficulty in swallowing small Brachyurous 

 z 86 as without great development of spines — for instance 

 Carcinus. 



A peculiar habit of the zotea at all stages is that of 

 frequently turning its abdomen backwards till the forked 

 telson reaches and embraces the dorsal spine, scraping 

 upwards as if to clean it. 'J'his action is so frequent that it 

 seems not to be connected with the process of moulting, 

 though possibly the stretching of the abdomen entailed may 

 assist in preparing for the act. Moulting seems usually to 

 take place at night, and must be a rapid process, for a 

 successful moult was never observed. Zoseas were fre- 

 quently found half freed from the larval skin, but these 

 specimens never succeeded in completely freeing themselves. 

 In fact, the new skin seems to harden so rapidly that unless 

 the process is completed at once failure results. 



The Megalopa stage is remarkably interesting from the 

 point of view of its habits. It has most of the characters 



VOL. 46, PART 3. — NEW SERIES. F V 



