WHITE SHRIMP FROM THE GULF OF MEXICO 



The protocephalon is clearly distinct from the 

 succeeding gnathothoracic and abdominal tag- 

 mata, and in Pendens sefiferus, and other species 

 of the genus (Grobben 1917) is independently 

 movable. Grobben, incidentally, described the 

 area as the "sincipit," a term apparently discarded 

 by Snodgrass in favor of the term, "proto- 

 cephalon." In the section below on the eyestalks, 

 a consideration of the muscles which move the 

 protocephalon on the gnathothorax will be found. 

 A discussion of the primitive order of the head 

 segments follows in the section on the labrum. 



SKELETAL ELEMENTS 



The protocephalon is constructed roughly on the 

 plan of a vertically placed hemisphere confluent 

 posteriorly with the gnathothoracic hemocoele. 

 The skeleton is flattened dorsally in the region of 

 the ocular plate (tig. 4) or lobe from which the fo- 

 ramina of the eyestalks are perforated. The large, 

 paired foramina of the antennules and antennae 

 and the mesal foramen of the labrum occupy al- 

 most the entire anterior and ventral surface of the 

 protocephalic skeleton. The latter regions are 

 heavily sclerotized and deeply indented between 

 tlic antennular and antennal foramina by the 

 medial stem of the Y-shaped epistome (figs. 28, 

 A ; 30) . The top, or posterior portion of the epis- 

 tomal Y, divides across the anterior face of 

 the labial foramen and the posterior side of the 

 large antennal foramina. Upon the lateral epis- 

 tomal bars, as shown by Snodgrass (1951), the 

 medial mandibular condyles are located. 



1. EYESTALKS 



Most of the following section on the eyestalks 

 was previously published by the writer (Young 

 1956). 



In some of the lower and in many of the higher 

 Crustacea, the compound eyes are set upon mov- 

 able stalks or peduncles. Their presence at the 

 ends of extensions has excited speculation for 

 many years. Oarcinologists have long discussed 

 the reasons for the eyestalks, their similarity with 

 other appendages (Caiman 1909), and the na- 

 ture of vision in a stalked-eyed animal, among 

 other things. Yet little has been written about the 

 mechanics of the eyestalk with respect to vision. 

 Xo one lias proposed any useful explanation for 

 the fine adjustments presumably available to a 

 compound eye which has as numerous oculomotor 

 muscles as the crustacean stalked eye. 



The presence of a set of muscles to move the 

 corneal surface of the compound eye on the eye- 

 stalk, and of muscles to move the eyestalk about 

 with respect to the body suggests the importance 

 of the position of the corneal surface relative to 

 the environment. By contrast, adjustments of 

 corneal position in an arthropod without eye- 

 stalks suggests a function of head and "neck" 

 muscles for activities other than feeding, if we 

 assume any importance to the arthropod of cor- 

 neal position adjustments. 



Recently, the eyestalk nerves of a few crusta- 

 ceans have been shown to contain neurosecretory 

 elements which evidently proliferate hormone 

 systems controlling such processes as molting 

 (Passano 1953). retinal pigment migration 

 (Welsh 1941), and chromatophore movements 

 (Perkins 1928). In view of the concentrated at- 

 tention currently being paid to matters of neuro- 

 hormonal control of physiological functions in 

 the arthropods, an understanding of the relation 

 of vision to neurosecretion appears to be near at 

 hand. 



The white shrimp, Pendens setiferus, carries its 

 eyestalks at an angle of about 75° to the median 

 sagittal plane and at an angle of about 10° to 15° 

 to a frontal plane at the ocular plate (fig. 4). 

 Only rarely are the eyes brought forward to lie 

 in the optic depressions of the antennules, and then 

 but for an instant for protection or possibly clean- 

 ing against the long plumules surrounding the 

 depressions. Normally, therefore, in P. setifer-us 

 and many other species of shrimps, the eyes and 

 stalks are widely spread and slightly upturned, a 

 situation not understood by morphologists who, 

 working with preserved materials, have described 

 the eyestalks as projecting anteriorly. Had pre- 

 vious workers taken into account the lateral posi- 

 tion of the eyestalk in the shrimps, and for that 

 matter in the crawfishes, a certain amount of con- 

 fusion in the naming of eyestalk musculature 

 might have been avoided. For in fact, medial 

 muscles are anterior and lateral muscles are pos- 

 terior. In the interests of uniformity, however, 

 certain of the incorrect names are here employed. 



P. setiferus is a bottom feeder. According to 

 an unpublished observation by Charles Dawson, 

 Bears Bluff Laboratories, Wadmalaw Island, 

 S. C, schools of penaeid shrimps are frequently 

 found on muddy bottoms. This worker describes 

 placing several P. aztecus Ives, 1891, in aquaria 

 with an inch or two of mud on the bottom, into 



