210 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.39. 



against the inside of the frontal margin. The opposite end is also 

 enlarged and fastened into the tissue just back of the mushroom. 

 The peg has disappeared, its function having been performed. The 

 eye of the larva is invisible externally, but it is probably present 

 inside the coils of the attachment filament in the same degenerate 

 state as at the close of the preceding stage. 



The epithelium of the mesenteron which began to be formed in the 

 metanauplius has now developed further and entirely surrounds the 

 mass of yolk globules. But the stomedeum and proctodeum are not 

 yet completed, and the larva is therefore incapable of taking nourish- 

 ment. Claus has described this first copepodid stage in Achtheres 

 percarum as possessing a completed digestive system, capable of 

 functioning. But he stands alone in such a statement. Nordmann, 

 KoUar, Vejdovsky, and Kellicott all represent the larval digestive 

 apparatus at this stage as only partially developed, and thus agree 

 with what is found in the present species. All the available accessory 

 testimony also witnesses to the same fact, that the larva at this time 

 is incapable of procuring or digesting food. 



The digestive system did not begin to develop until the latter part 

 of the preceding stage and did not advance very rapidly. At the 

 close of the stage when the larva was ready to molt into the copepodid 

 form, the mesenteron was not yet completed while the stomodeum 

 and proctodeum were scarcely begim. It would be impossible for all 

 three to be finished and ready to function in the short time consumed 

 in molting. Again, the mouth-tube is not yet completed. The halves 

 of the under lip, which are to form the sucking tip of the organ, have 

 not yet grown together and could not, therefore, perform their 

 destined function. Moreover, the mandibles are still on the outside 

 of the proboscis and in a very rudimentary state, entirely unfitted for 

 piercing. This combined testimony practically proves that theiarva 

 is nourished by the abundance of yolk still left in its stomach until 

 after it has attached itself to a host. 



SEARCH FOR A HOST. 



In its search for a host the larva swims about actively, the long 

 rowing setae furnishing powerful locomotor organs. The motion is 

 not at all jerky like that of the free-swimmers, but is smooth and rapid 

 like that of an Argulus or Caligus. In an aquarium this energy is 

 kept up for about twenty-four hours and then relaxes, the larva by 

 the second day becoming so wearied that its feeble efforts scarcely 

 move it at all. In fact, it has been the common experience of all 

 investigators that such larvae usually die within thirty-six hours 

 unless thay find a host. This fact, combined with the complete 

 development of the attachment filament and the rudimentary and 



