454 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



All those organs whose growing primordia so encumbered the zoea 

 are now developed and functional. The megalops is about 3 milli- 

 meters long. It is easily seen as it darts about the surface. The 

 sensory organs are now well developed. The eyes are large and well 

 formed. In the base of the antennule there is a statolith, and the 

 animal swims about in any direction and can change direction quickly 

 and accurately. It is no longer guided in its movements solely by 

 its reaction to light, but is independent of this tropism and seeks 

 its prey at all depths. It remains at the surface most of the time, 

 however, as food is most abundant there. Besides the statoliths 

 and eyes other sense organs are now better developed then in the zoea. 

 Both the antennule (fig. 20) and antenna (fig. 22) bear numerous 

 hairs that are tactile and also others that are the organs of the 

 chemical sense, taste, or smell. The animal can hear, in the sense that 

 its delicate hairs perceive the vibrations in the water just as the ears 

 of higher animals record the sound waves of the air. These " hearing " 

 hairs are especially located on the antennae and antennules. The 

 antennae now have lost the spine and the exopodite of the zoea, and 

 only the flagellum remains. The first two pairs of appendages are 

 practically in their adult crab condition, although still minute, of 

 course. 



The shape of the body is now more that of a crab than that of a 

 zoea, but in some respects it is intermediate between the two. The 

 spines of the zoeal carapace are lost and the body is somewhat flat- 

 tened from above and below. However it is still considerably 

 longer than broad and in this respect resembles somewhat the body 

 of a shrimp. In fact the megalops may be said in general to have 

 the body of a shrimp or crayfish with the legs of a crab. The 

 abdomen is like that of the crayfish and in swimming is carried 

 extended straight out behind. When the animal comes to rest, how- 

 ever, it is folded under the body and the rnegalops then looks very 

 much like a tiny crab. 



The zoeal mouth parts are very little changed in the megalops. 

 They become larger and more hairy. The mandible (fig. 23) shows 

 one considerable change in accordance with the development of 

 all the sense organs, it now bears a palp, which is an organ of 

 taste and of touch. In addition to the mouth parts of the zoea the 

 megalops has three other pairs of mouth parts. The first (fig. 24) 

 and second (fig. 25) maxillipeds are no longer organs of locomo- 

 tion. Their exopodites are not now their main functional parts, 

 but on the contrary form rather slender palps that probably have 

 merely a tactile function. The basal portions of the appendages 

 are greatly enlarged and along their median borders bear stout 

 hairs. The endopodites are enlarged and muscular. Their proxi- 

 mal segments bear stout hairs along their median borders and with 



