448 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



that resemble the newly hatched crab larvae, just as caterpillar de- 

 scribes a certain form of larva among the insects. 



The body of the zoea (figs. 1 and 8) is divided into two regions, 

 cephalothorax and abdomen, just as is that of the adult fiddler. 

 The cephalothorax of the larva, however, is very different from that 

 of the adult. Instead of being flattened from above downward it 

 is flattened from side to side as are the bodies of many other swim- 

 ming forms, e. g. shrimps. It is covered by a shield or carapace 

 dorsally and laterally. Laterally the carapace extends downward 

 over the bases of the appendages. Posteriorly it has a broad, deep 

 notch, through which protrudes the abdomen. At just about its 

 central point, dorsally, the carapace is produced upward into a 

 stout, slightly recurved spine. (Fig. 1, a.) Another spine of this 

 kind extends ventrally from the anterior margin of the carapace. 

 (Fig. 1, b.) It is. difficult to imagine how these spines may be of 

 use to the zoea. Blood circulates through them and it may be aerated 

 in its passage. On the other hand the spines often become entangled 

 in floating debris or algae and the larva is unable to free itself. What- 

 ever may be their service to the zoea, however, the spines are of 

 definite use to the scientist. They enable him to distinguish the 

 zoea of Uca from that of most other crabs. Most crab larvae have 

 a dorsal spine, an anterior spine and two lateral spines on the 

 carapace. The zoeas of the fiddler crabs and some of their relatives 

 lack the lateral spines. 



The abdomen of the zoea is a segmented cylinder. It is divided 

 into five parts, and each of these is a simple, short cylinder, except 

 the terminal segment. This is produced posteriorly into two long 

 laterally diverging horns. The abdomen is carried flexed under 

 the cephalothorax in swimming, although not pressed up against 

 it as in the adult. It is not used as an organ of locomotion, but 

 when the zoea comes to rest on the bottom, or is swept against an 

 obstruction of any kind, the abdomen lashes out and about, and 

 the larva squirms and wriggles in very lively fashion. 



On each side of the anterior part of the cephalothorax there is a 

 large compound eye. These are not borne on stalks as in the adult 

 and they can not be moved in any way. They resemble in this respect 

 the eyes of such lower Crustacea as the water-fleas (Talorchestia), 

 pill-bugs (Oniscus), and Cyclops. The color of the eyes is dark 

 green, almost black. In spite of the large size of the eyes, the zoea 

 can not see in the sense of distinguishing shapes. It can, however, 

 differentiate different intensities of light. This ability makes pos- 

 sible one of the most important tropisms of the larvae. They always 

 swim toward a region where the light is brightest. This serves to 

 keep them at the surface of the sea in spite of the fact that they 

 have no organ of equilibration. When the zoea bursts from its egg 



