WHITE SHRIMP FROM THE GULF OF MEXICO 



17 



of the basal segment. The muscle runs the length 

 of the eyestalk to insert in connective tissue near 

 the dorsal calathus retractor muscle. Contraction 

 of the muscle swings the eyestalk horizontally to 

 a lateral position. The eyestalk abductor muscle 

 of Penae.Wi is very likely homologous with the ab- 

 ductor muscle described for Astacus and Pandalus, 

 and possibly with the lateral branch of the ocular 

 abductor muscle in C'allinectes. 



CALATHUS RETRACTOR MUSCLES 



The muscles in Penaeus which retract the optic 

 calathus appear to be clearly represented by the 

 retractor muscles of the eyes of Astacus, I 'ambarus 

 barton/ ( Fabricius 1798). Pandalus, and Cal- 

 linectes. Phylogenetically, the situation in Pe- 

 naeus is somewhat more generalized than in the 

 other forms which we are considering, in that sev- 

 eral of the calathus retractor muscles in Penaeus 

 have more than one part. In addition, Penaeus 

 has a number of apparently independent rotator 

 muscles, none of them previously described, which 

 function to twist the optic calathus about a longi- 

 tudinal axis through the eyestalk. 



DORSAL CALATHUS RETRACTOR MUSCLE 



Figures 6. 7, 8 



The dorsal calathus retractor muscle arises in 

 connective tissue near the ventral surface of the 

 eyestalk and attaches to the dorsal edge of the 

 calathus. 



LATERAL CALATHUS RETRACTOR MUSCLE 



Figures 6, 7, 8, 9 



The lateral calathus retractor muscle, really the 

 posterior retractor, originates on sclerotized mate- 

 rial along the lateral, or actually posterior, blood 

 simls running the length of the eyestalk. The 

 larger portion of this muscle attaches on the 

 lateral edge of the calathus, the lesser part turning 

 ventrally and running across the ventral edge of 

 the calathus, just dorsal to the ventral retractor 

 muscles (fig. 9). When this muscle contracts it 

 not only retracts the calathus, but rotates the cal- 

 athus about an. axis longitudinal to the eyestalk. 



VENTRAL CALATHUS RETRACTOR MUSCLE 



Figure 9 



The ventral calathus retractor muscle originates 

 on several sclerotized regions on the ventral sur- 



face of the eyestalk. One part of the muscle is 

 long and slender, while the others are short and 

 arise from broad origins (fig. 9). The muscle is 

 inserted over a wide area on the ventral edge of 

 the calathus. 



MEDIAL CALATHUS RETRACTOR MUSCLE 



Figure 6 



The medial calathus retractor muscle originates 

 on two points in the region of the median tubercle, 

 and actually is comprised of two muscles (fig. 6). 

 The larger muscle originates in the median tu- 

 bercle and inserts in connective tissue dorsal to 

 the distal optical ganglionic mass. The smaller 

 muscle originates dorsal to the larger muscle and 

 inserts on a ventromedial point on the calathus. 

 The contraction of both muscles results in medial 

 retraction of the calathus; functioning in opposi- 

 tion, the muscles retract the calathus in a vertical 

 plane, reinforcing the action of the dorsal and 

 ventral retractor muscles. 



CALATHUS ROTATOR MUSCLES 



Figures 6, 7 



At least three calathus rotator muscles may be 

 seen in the eyestalk of Penaeus setiferus. Rotator 

 muscles of this type have not been described for 

 Pandalus, Astaevs, Cam barus. or C'allinectes. The 

 calathus rotators bear a certain similarity to one 

 another, in that they are all superficial in position 

 and insert in the heavy connective tissue under- 

 lying the thick cuticle of the calathus. 



2. ANTENNULES 



The antennules, or first antennae, are said to 

 belong to the second segment of the body of Crusta- 

 cea, following the eyes. In the company of the 

 eyes, the antennae, and the labrum, the antennules 

 are attached to the body tagma that has been term- 

 ed by Snodgrass (1951a: 1952) the protocephalon. 

 Whereas the appendicular nature of the eye- 

 stalk generally has been questioned, the status 

 of the antennules as true crustacean appendages 

 has rarely been attacked, despite controversy over 

 the homologies of the component segments. 



The antennule of Penaeus setiferus is comprised 

 of a proximal stem divided into three basal seg- 

 ments, called the protopodite by Huxley (1906), 

 and two distal flagella. Proponents of theories 

 such as Huxley's (1906) suggesting correspond- 

 ence of parts between the segments of the anten- 



