24 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



gations show that the flagella are involved in 

 chemoreception (Doflein 1911; Balss 1913; Bell 

 1906; Lissman and Wolsky 1935; Spiegel 1927). 

 The olfactory nature of the antennular flagella is 

 related to an interesting structural modification. 

 This modification is the apparent sexual dimor- 

 phism of the medial flagellum in the adult male 

 of Penaeus set! fetus (fig. 17). The change to the 

 medial flagellum, which is easily visihle to the 

 naked eye, results in a pronounced dorsoventral 

 flattening, together with the appearance of nu- 

 merous, stout processes of two sizes on the dorsal 

 margin. That these processes may be predomi- 

 nantly proteinaceous is suggested by their loss fol- 

 lowing treatment of the preserved material in 

 strong alkali, for as is well known strong alkali 

 degrades the glucosamine of chitin but does not 

 immediately remove it, whereas the alkali-soluble 

 proteins of the internal organs and cuticle are 

 rendered soluble and washed out (Richards 1951). 



Other changes to the condition of adult male- 

 ness probably occur during the same molt that 

 modifies the medial flagellum. The most obvious 

 secondary sexual character is the mesial joining 

 of the previously free wings of the modified 

 pleopod endopodities of the first abdominal seg- 

 ment to form the definitive and functional pe- 

 tasma, or sperm-transfer organ. 



Since the antennular flagella are olfactory, the 

 sexually dimorphic medial flagellum of the male 

 Penaeus setiferus probably functions to enable the 

 male to find the sexually mature female during 

 the time of mating. The occurrence of secondary 

 sexual modifications of the antennular flagella are 

 widespread among the Crustacea Decapoda. 

 Meredith (1954) describes a modified outer anten- 

 nular flagellum in the adult male of Crangon 

 vulgaris and suggests that the structure may func- 

 tion in mating. This worker finds that the 

 character is of use in identifying adult males in 

 the field. The most extensive treatment of the 

 subject of sexual dimorphism in Crustacea is that 

 of Rioja (1939a, 1939b, 1940a, 1940b, 1940c, 1941a, 

 1941b, 1942a, 1942b). who has described in detail 

 the sexually modified medial flagellum of Penaeus 

 setiferus and several other species of penaeid 

 shrimps. This worker considers the character to 

 be constant in occurrence and sensory in function, 

 and closely related to the sexual activities of the 

 shrimps. 



In the opinion of the present writer, the modi- 

 fied medial antennular flagellum is a constant 



character in the sexually mature male of Penaeus 

 setiferus. However, the subject needs further 

 study. A large, statistically significant number 

 of male and female Penaeus setiferus shown to be 

 sexually mature by the histological methods of 

 King (1948) should be examined to prove the 

 point. In the present study 10 adult males and 

 8 adult females were considered in connection 

 with the character. 



MUSCLE ELEMENTS 



The antennule of Penaeus setiferus contains 13 

 muscles, as contrasted with 12 for the caridean 

 PandaJus. and 9 for the antennule of Astacus. 

 Cochran (1935) lists 9 for Callinectes, includ- 

 ing a double antennular promotor muscle and a 

 double antennular remotor muscle. If these di- 

 vided muscle bodies are actually 2 promotor and 

 2 remotor muscles, then the antennule of Calli- 

 nectes can be said to have 11 muscles. 



ANTENNULAR ABDUCTOR MUSCLE 

 Figure 14 



The antennular abductor muscle of the first an- 

 tennular segment is attached posteriorly to an 

 apodeme that arises from the ventrolateral mar- 

 gin of the articular foramen by which the anten- 

 nule is connected to the protocephalon. The plane 

 of orientation of this apodeme is vertical. The 

 antennular abductor runs anteriorly a brief dis- 

 tance and divides into two large branches that at- 

 tach to sclei'otized bars supporting the posterior 

 region of the first antennular segment in the 

 neighborhood of the statocyst. Contraction of 

 the first segment abductor muscle swings the first 

 antennular segment, and with it the distal anten- 

 nular segments, outward from the mid-sagittal 

 plane of the body in the limited horizontal motion 

 possible to the first segment. The antennular ab- 

 ductor of Penaeus is homologous with the nius- 

 culus promotor I antennae of Astacws, Pandalus, 

 and possibly to the first segment promotor mus- 

 cles of Callinectes, judging from the origins, in- 

 sertions, and arrangements of the muscles. 



However, the functions of the homologous 

 muscles are different, as often happens. The dif- 

 ference lies primarily in the restriction of anten- 

 nular movement created by the presence of the an- 

 tenna! scales ventral to the antennules in Penaeus. 

 In the white shrimp, the scales are relatively large 

 and, when lying in their usual longitudinal posi- 



