30 



FISHERY BULLETIN OF THE Fl jH AND WILDLIFE SERVICE 



3. THE ANTENNAE 



The antennae, or as they are frequently termed, 

 the second antennae, display wide variations 

 among the Crustacea. In certain of the Copepoda 

 and other groups in the lower Crustacea they are 

 large swimming organs. They are modified for 

 clinging in other copepods. Frequently the an- 

 tenna of the male crustacean is modified as a 

 sexual clasper. Sexual dimorphism of the an- 

 tennae is widespread in the lower Crustacea, the 

 appendage of the adult male being more highly 

 developed than that of the female. In several 

 groups the structures may be extremely reduced 

 or even lost in the adult animals. 



The antenna of the higher Crustacea is fairly 

 uniform, although exceptions do occur. The 

 malacostracan antenna is said to consist of a 2- 

 or 3-segmented protopodite, an endopodite of 2 

 or 3 segments, the distal-most bearing a flagellum, 

 and an exopodite reflected into a flat scale. The 

 protopodite usually is comprised of the coxopo- 

 dite and basipodite, but some forms may have a 

 proximal pre- or sub-coxa in addition (Caiman 

 1009). Carcinologists use the expression "pe- 

 duncle" to refer to the total number of endopodite 

 and protopodite segments, usually 5 or 6, in the 

 Malacostraca. Representatives of the lower De- 

 capoda, like Penaeus setiferus, have a 5-seg- 

 mented antenna] peduncle, made up of the 

 coxopodite and basipodite in the protopodite, and 

 3 proximal endopodite segments. The exopodite 

 is scalelike. The scale is often missing or reduced 

 to a spine in the higher decapods and the endopo- 

 dite and flagellum may be relatively small. 



Functionally, the antennae have always been 

 thought to be sensory. More specifically, the 

 antenna! flapella are said to be centers of tactile 

 sensation, whether pressure or simple touch is not 

 known. No experimental data to support these 

 contentions exist. The functions of the antenna! 

 scale would appear to be varied, in accordance 

 with its great variation in size. The scale is 

 missing in the Brachyura and small in the craw- 

 fishes and other Astacura, making its function 

 somewhat difficult to determine by observational 

 means. The organ is very large in the swimming 

 decapods of the Natantia. The suggestion has 

 frequently been made that the scale is an anterior 

 swimming plane in the hitter group. 



Some attempt has been made to ascertain the 

 function of the antenna] scale in the swimming 



of the white shrimp. Normally the scale is car- 

 ried in a directly anterior position, rotated 

 slightly on its longitudinal axis. The rotation is 

 such that the thin mesial margins of the scale lie 

 somewhat ventrad of the heavy lateral margins. 

 The effect produced is that of a ship's bow. 

 Shrimps in an aquarium were lightly secured by 

 a loop of string to the end of a rod. Lifted from 

 the bottom in this position, the animals are stim- 

 ulated to swim forward. Jets of water from a 

 pump were directed at the motionless shrimps 

 from various directions to see whether the an- 

 tennal scale was used as an anterior steering de- 

 vice. No compensatory movements of the scale 

 were observed. The organs appeared simply to 

 cleave the water ahead. During normal move- 

 ments about an aquarium, the scales are occasion- 

 ally spread widely, but are never kept in the 

 spread condition for more than a moment. 



The antenna in Penaeus setiferus is typical of 

 the natant Decapoda. The antenna] scale is broad 

 and strong and extends as far as the anterior tip of 

 the rostrum. The protopodite is comprised of two 

 segments, a short, incomplete coxopodite and a 

 very large basipodite. To the large basipodite is 

 articulated the exopodite, or scale, and the basal 

 segments of the antenna. The size of the basi- 

 podite reflects its support of the heavy scale rather 

 than that of the smaller antenna] segments. The 

 long antennal flagellum is carried laterally, from 

 which position the movements of the shrimps 

 through t lie water cause the flagellum to drag 

 alongside and some distance behind the animal. 

 The flagellum is approximately twice the body 

 length in the white shrimp. 



SKELETAL ELEMENTS 



The first antennal segment, or coxopodite (fig. 

 18), is an incomplete ring by which the antenna 

 is attached to the protocephalon. The foramen of 

 tli-.' antenna at which the first antennal segment 

 articulates with the protocephalon is by far the 

 largest of the head foramina. At its broadest 

 portions, muscles insert upon the first antennal 

 segment. 



The basipodite, or second antennal segment 

 (figs. IS, 23), is a large, heavily sclerotized article 

 connected firmly to the first antennal segment 

 proximally and articulating distally with the third 

 antenna] segment, or ischiopodite, of the endopo- 

 dite, and with the antennal scale, or exopodite. At 



