450 A2STNTJAL REPORT SMITHSONIAN INSTITUTION, 1920. 



three pairs of appendages act together as very effective instruments 

 for securing food. The scaphognathites beat rapidly and contiuously 

 in such a way as to draw a stream of water backward past the mouth 

 and along the bases of the mouth parts. As the zoeas live in the 

 thickly populated surface area of the sea, this stream of water will 

 contain some of the small animals and plants that abound there. 

 When such an animal touches the hairs of one of the tactile palps 

 the spiny lobes of the maxillules and maxillae close in upon the prey 

 and secure it for the mandibles. It is pushed up between these and 

 they crush it. There is a hairy lip in front of the mouth (fig. 1, d) 

 and another behind it. These serve to guide the food to the mouth 

 opening. 



The two pairs of thoracic appendages that are present in the 

 zoea are the first and second maxillipeds. (Fig. 9.) The absence 

 of the third maxillipeds serves to distinguish the zoeas of the 

 higher crabs (Brachyura) from those of the lower crabs (Ano- 

 mura) and shrimps (Macrura) in which the third pair of maxilli- 

 peds are also present. The maxillipeds of the zoea are very dif- 

 ferent from those of the adult, both in structure and in function. 

 Each is composed of a basal segment bearing two distal rami. 

 The outer ramus, or exopodite, is a simple rodlike segment which 

 bears four long plumose hairs at its tip. The inner ramus, or en- 

 dopodite, is a segmented cylinder. It bears a few tactile hairs scat- 

 tered along its segments and the terminal segment bears a short 

 plumose hair that may possibly be " auditory " in the sense that it 

 enables the zoea to perceive vibrations in the water. The endopo- 

 dite of the first maxilliped has five segments while that of the sec- 

 ond had only three. 



The chief function of the zoeal maxilliped is locomotion. The 

 exopodites beat rapidly up and down in such a way as to drive the 

 little animal upward and forward. Each of the plumose swim- 

 ming hairs is jointed near its middle so that it bends on the up 

 stroke and straightens on the downward beat. The zoeas swim 

 actively nearly all the time. Their efforts, however, are chiefly to 

 keep at the surface. The tides serve to carry them from place to 

 place. As the zoeas are hatched on an ebb tide most of them will 

 be swept away to sea in a few hours and by dawn they will be 

 widely scattered. Locomotion is not the only function of the 

 maxillipeds. The endopodites are organs of touch and possibly 

 of " hearing." The entire body of the young zoea is covered 

 by a thin layer of chitin that is tough and inelastic. There 

 has not yet been any calcification in the chitin and so it is very 

 transparent. Many of the internal organs may be seen easily. The 

 heart lies just beneath the base of the dorsal spine. It beats rap- 

 idly but quite irregularly, stopping for a minute or two at times. 



