58 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I 



or to its own future adult stage. Such nonancestral forms are par- 

 ticularly evident in parasitic species. 



Gumey (1942), referring to the progressive shift of the swimming 

 function in the larva from the antennae to the pereiopods and finally 

 to the pleopods, has expressed the idea that "the fundamental fact 

 which determines the organization of the larva is the mode of locomo- 

 tion." However, it is to be presumed that the use of the pleopods for 

 swimming was first established in the adult ancestors of such species. 

 The nauplius naturally cannot swim in this ancestral manner, and 

 must use what appendages it has. As the larva grows by the addition 

 of segments and appendages it can more efficiently swim by making 

 use of the pereiopods, and finally when the pleopods are developed it 

 can swim in the adult manner. It is the progressive organization of 

 the larva, therefore, that determines the mode of locomotion. 



The Macrura. — The macruran decapods are the lobsters and the 

 crayfishes. The lobster, Homarus, according to S. I. Smith (1871- 

 1873) undergoes its early development in the tgg and hatches at a 

 stage when all the pereiopods are present and are equipped with 

 feathery exopodites. This first free stage of the lobster (fig. 22 A), 

 therefore, corresponds with the mysis stage of Pcnaeus (fig. 21 E). 

 In the next instar the larva increases somewhat in size, and rudiments 

 of pleopods appear on the abdomen. In the third instar (fig. 22 B) 

 the young lobster attains a length of 12 to 13 mm. and much re- 

 sembles the adult; the chelae are well developed, the pleopods are 

 biramous, but the exopodites are still present on the pereiopods. 

 Smith suggests that there is probably another instar intervening be- 

 tween tlie third and the adult when the exopodites are lost, as in the 

 postmysis of Penaeus (fig. 21 F). 



The fresh-water crayfishes, Astacus and Camharus, hatch at a later 

 stage of development than Homarus, when they have practically the 

 adult structure except for the lack of the first and sixth pleopods. 



The Brachyura. — The brachyurans, or "short-tailed" decapods, are 

 the ordinary crabs, so named because of the small size of the abdomen, 

 which in the adult is carried bent forward beneath the thorax. The 

 zoeal lar\-ae are characterized in most species (fig. 23) by the presence 

 of a long dorsal spine on the thorax and by the spinelike form of the 

 rostrum, the two often projecting in a straight median line from 

 opposite ends of the back. Some have also lateral spines. The larva 

 swims with the large first and second maxillipeds, and the spines are 

 supposed to assist in directing the course of the larva in the water 

 or to help keep it afloat. The spines are absent in only a few species, 

 as in the genus Ehalia and in members of the Pinnotheridae. The last 



