20 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



1 



A 



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Figure 11. — Dorsal view of left eyestalk. Diagrammatic. 

 The skeletal bars of the eyestalk are so arranged that 

 the position of the ommatidial surface is maintained 

 whatever the angle of the eyestalk with respect to the 

 long axis of the shrimp. 



nule and those of the typical crustacean appendage 

 have not received support. Most carcinologists 

 consider the parts of the antennule so extensively 

 modified from the usual plan as to defy identifi- 

 cation. At the same time, the clearly segmented 

 structure of the antennule marks it as a true ap- 

 pendage, in the opinion of many workers. 



The form and function of the crustacean anten- 

 nule usually is said to be constant among the 

 Decapoda, relative to the extraordinary vari- 

 ability in the appendage occurring in the lower 

 Crustacea. Nonetheless, among the decapods 

 wide differences are found. The outer or lateral 

 flagellum is split in many of the Caridea, giving 

 the impression that the antennule bears three 

 flagella. Among the tribe Penaeidae, the outer 

 flagellum is prehensile in certain genera of the 

 Family Sevgestidae. In the Solenocera each 

 flagellum is semitubular, thus forming a siphon 

 (Caiman 1909). The antennule is small and re- 

 duced in the Brachyura. 



Despite infrequent anatomical treatment of the 

 decapod antennule, the available work suggests 

 that a marked uniformity exists in the appendage 

 and its parts over a broad spectrum of the Deca- 

 poda. From the standpoint of comparative mor- 



phology this feature of similarity would appear 

 to simplify the process of homologizing part to 

 part, and does, except for the matter of muscle 

 names, a problem discussed earlier. 



SKELETAL ELEMENTS 



In dorsal view the antennule is shaped in a long 

 wedge pointing anteriorly, the broadest portion 

 being the proximal part of the first segment at the 

 region of attachment of the antennule with the 

 skeleton of the body. Each successive distal seg- 

 ment gradually becomes narrower. The mesial 

 edge of the basal segments is straight and flat 

 where the appendage rests against the other an- 

 tennule. The first, or proximal, antennular seg- 

 ment (fig. 12) is relatively larger and more com- 

 plex than the segments distal to it. The first seg- 

 ment is attached by a large articular foramen to 

 the protocephalon, the tagma to which the anten- 

 nules belong. Here, dorsal and ventral points of 

 articulation, or condyles, afford the first anten- 

 nular segment limited horizontal movement. 



Centered in the posterior region of the first an- 

 tennular segment is the statocyst, the organ that 

 is thought to mediate the special sense of equili- 

 bration (figs. 12 to 16). Earlier workers termed 

 the structure an otocyst, having ascribed to it an 

 auditory function (Huxley 1906). The statocyst 

 is constructed like an incomplete sac that has 

 dorsal and anterior openings. Hairs, presumably 

 sensory, project from the inner surface of the 

 cuticular sac into the lumen of the statocyst. The 

 hairs are arranged in regular rows. The rows of 

 hairs are confined to oddly shaped patterns lo- 

 cated at various places on the inner surface of 

 the statocyst. Nerves from the sensory hairs 

 coalesce ventrad of the sac into a broad, flat nerve 

 tract that enters the brain at a point ventral to 

 the optic tract (fig. 14). 



The dorsal opening of the statocyst is partially 

 covered by a fleshy, heavily setose lobe, the dorsal 

 closing lobe (figs. 12, 15). In fresh material the 

 dorsal closing lobe may be lifted from the dorsal 

 opening of the statocyst without great difficulty. 

 The saccular statocyst is shut anteriorly by a thin, 

 curled sheet of cuticle that arises vertically by 

 evagination from the ventrolateral floor of the an- 

 tennular eye depression. The sheet is designated 

 here as the anterior closing plate (figs. 12 to 15). 

 Neither the dorsal closing lobe nor the anterior 

 closing plate effect complete obturation of the 

 statocyst. As a result, water circulates through 



