8 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



which the animals immediately burrow, except for 

 the eyes. Such behavior suggests that the long 

 eyestalks are among the organs enabling the pe- 

 naeid shrimp to make use of mud for protection, 

 especially after molting. 



In the past, observers have described square 

 corneal facets in the eyes of several species of 

 decapod crustaceans (Huxley 1906; Caiman, 

 1909). A study of slides made of the corneal 

 cuticle shows that the corneal facets in the com- 

 pound eye of Penaeus setiferus are also square. 

 Likewise, the underlying crystalline cone cells are 

 square in the white shrimp and total four per om- 

 matidium, as determined by the study of tangen- 

 tial sections of the eye from which the corneal 

 cuticle had been removed. Ramadan (1952) re- 

 ports a similar situation in a species of Metap'e- 

 naeus. In longitudinal section the ommatidium of 

 P. setiferus is like that of Astacus, as described by 

 Bernhards (1916), with comparatively elongate 

 crystalline cone and short rhabdom cells. If any- 

 thing, the cone cell is longer in the white shrimp 

 than in the crawfish. A light pink substance 

 which gives the dark-adapted shrimp eye its 

 bright color in strong light is the tapetum (Ram- 

 adan 1952). It is associated with the proximal 

 or retinal pigment of the ommatidia. 



SKELETAL ELEMENTS 



The ommatidial surfaces arise from a sclero- 

 tized cup, previously named the optic calathus, or 

 basket (Young 1956), to avoid confusion with the 

 optic cup of the vertebrate embryo (fig. 4). The 

 optic calathus rests upon the elongate stalk seg- 

 ment in a structural relation permitting univer- 

 sal movements, although the degree of movement 

 varies in different planes. 



Two points of articulation in the dorsoventral 

 plane allow the optic calathus considerable hori- 

 zontal movement around the distal end of its sup- 

 porting stalk. These dorsoventral hinges are, 

 however, sufficiently loose to permit vertical and 

 rotational calathus movements, but to a lesser ex- 

 tent than horizontal movements. The long stalk is 

 comprised externally of several longitudinal 

 sclerotized bars which are separated by pliable 

 cuticle. Two of the bars give support to the dorso- 

 ventral points of articulation (fig. 11) and others 

 to less well-defined points of articulation between 

 the stalk and calathus, and between the stalk and 

 basal segment. 



The stalk is movable upon the short, boxlike, 

 basal segment in the horizontal plane. Vertical 

 movements between the basal segment and the 

 stalk are restricted. With respect to the structure 

 here labelled the median tubercle (fig. 4), Ander- 

 son and Lindner (1943) and Voss (1955) state 

 that shrimps of the subfamily Penaeinae have no 

 distinct median tubercle on the ocular peduncle. 

 However, many of the shrimps of this group do 

 possess large, blunt, median tubercles, similar to 

 those in Penaeus setiferus. 



Set between the basal segments is the ocular 

 plate or lobe, a broad, roughly rectangular sclero- 

 tized structure which encloses laterally the medial 

 parts of the basal segments (fig. 4). The ocular 

 plate is the dorsal-most region of the protoce- 

 phalon. Movements between the basal segment 

 and the ocular plate are similar in extent to those 

 between the stalk and the basal segment. Hori- 

 zontal movements are limited to an arc of about 

 15° or 20°. 



MUSCLE ELEMENTS 



PROTOCEPHALON MUSCLES OF THE OCULAR 

 REGION 



Taking origin from either the epistomal invag- 

 ination or the dorsal surface of the carapace and 

 inserting upon basal parts of the eyestalks are 

 four pairs of muscles. The basal regions of the 

 eyestalks will be assigned here to the dorsal part 

 of the protocephalon. 



ANTERIOR PROTOCEPHALON LEVATOR MUSCLES 

 Figures 34, 35 



The tiny anterior protocephalon levator muscles 

 are probably the muscles designated by Grobben 

 (1917, 1919) as the protocephalon levators (mus- 

 culus levator sincipitis) in a European penaeid 

 and in other species of higher Crustacea. These 

 muscles are difficult to make clear, either by dis- 

 section, or by illustration, since they take origin 

 on the carapace, on the nearly vertical sides of the 

 rostral base. During removal of the carapace and 

 the underlying layers of tough, fibrous epidermis 

 and connective tissue, these muscles are torn 

 away. The anterior protocephalon levators insert 

 in the heavy connective tissue associated with the 

 posterior edge of the protocephalon. Their actual 

 levation of the protocephalon is negligible, since 

 they are not only minute in cross section, but 



