22 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



the statocyst at all times, to the extent that the 

 heavy investment of hairs in the area permits. 



In living shrimps, as has been shown by Burken- 

 road (1939), the open statocyst contains sand 

 grains. The sand probably functions as statoliths, 

 if the statocyst is indeed equilibratory. These 

 statoliths are obtained by the shrimp from the sub- 

 strate through activities of the animal that are 

 either directed or incidental to their collection. 

 Cursory examination of the statocyst does not re- 

 veal bodies that have obviously been secreted by 

 the shrimp. Burkenroad (1939), however, finds 

 that the statocyst of penaeids contains statoliths 

 secreted by the animals. Since the statocyst is 

 open to the water and to the substrate into which 

 the shrimp is known to burrow, the possibility 

 exists that the statocyst contents may undergo con- 

 tinual replacement during intermolt periods. 

 However, since the shrimp loses the statocyst with 

 its contents at each ecdysis, the statoliths are prob- 

 ably replaced in large measure at the time. That 

 the animals burrow into the substrate for protec- 

 tion, statolith replenishment, and rest following 

 the molt is supported by negative information 

 provided by observing shrimps in clean aquaria at 

 ecdysis. Upon shedding in this unnatural environ- 

 ment, the shrimp is unable to navigate properly 

 and soon perishes, even if kept alone. The stato- 

 eysts are found to be empty of statoliths. 



The largest region of the first antennular seg- 

 ment is the eye depression. Beginning at the 

 anterior closing plate and extending to the mar- 

 gins of the first antennular segment distally and 

 to the sides is a broad, deep concavity into which 

 the corneal surface of the compound eye may rest 

 (figs. 12, 13). The eye depression is confined 

 proximally by the anterior closing plate of the 

 statocyst and the skeletal structures that surround 

 the statocyst. The dorsal closing lobe projects out 

 slightly, dorsal to the depression. A large, wedge- 

 shaped, fleshy lobe arising in the posterolateral 

 region of the first segment extends anteriorly along 

 the lateral margin to a point. At the anterolateral 

 corner of the first antennular segment is a small, 

 sharp stylocerite (fig. 12), a structure common 

 to the Tribe Penaeidae (Voss 1955). Along the 

 mesial margin of the first segment lies the dorsal 

 eye brush, or prosartema (figs. 12, 13), a long, 

 thin lobe arising dorsally from the proximal re- 

 gion of the segment and extending anteriorly to 

 the anteromesial corner of the segment. 



Most of the structures lining the eye depres- 

 sion are heavily setose, to a degree that would 

 make their outlines obscure if illustrated faith- 

 fully. For this reason only a fraction of the 

 true covering in hair of the dorsal side of the 

 antennule has been shown. The presence of this 

 extensive investment of hair probably can be 

 explained as a system of brushes to clean the 

 corneal surfaces of the compound eyes. Many 

 stalk-eyed crustaceans carry the eyes laterally 

 and never for any length of time in the eye de- 

 pressions of the antennules. However, the eyes 

 are brought frequently into and out of the de- 

 pressions, and thus through the long hairs lin- 

 ing the depression. 



The second antennular segment (fig. 12), a far 

 simpler structure than the first segment, is at- 

 tached to the latter distally. The second seg- 

 ment articulates with the first segment allowing 

 limited horizontal movements of the antennular 

 segments distal to the first segment. The sec- 

 ond segment is a rectangular box in shape and 

 modified for the actions of the muscles which it 

 contains. 



The third antennular segment (fig. 12) is a 

 small, square structure articulated with the sec- 

 ond segment. Movement of the third segment 

 and the flagella on the second segment is limited 

 to an attenuated horizontal arc. No special sense 

 organ appeal's in the third antennular segment. 



Two flagella, the medial flagelluni and the lat- 

 eral flagellmn (fig. 12), articulate independently 

 with the third antennular segment. The points 

 of articulation between the flagella and the third 

 segment are so arranged that the flagella may 

 move through a broad arc in the frontal plane. 

 Each articulates with the third segment in 

 slightly different horizontal planes. The lateral 

 flagellum is attached dorsad of the medial flagel- 

 lum. The flagella are composed of many short 

 articles of light construction connected by rings 

 of thin cuticle, thus permitting bending in all 

 planes. The length of the articles differs in the 

 two flagella, those of the lateral flagellum being 

 shorter than the articles of the medial flagellum. 

 Other differences between the flagella include 

 variation in cross-sectional shape and in the 

 types of setae and processes projecting from the 

 articles. 



Numerous studies have been made on the func- 

 tion of the antennular flagella and the variety of 

 hairs and processes that they bear. These investi- 



