10 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



short in length. No counterpart of the anterior 

 protocephalon levator muscles was described by 

 Schmidt (1915) in Astacus, or by Berkeley (1928) 

 in Pandalus. However, Grobben (1917) illus- 

 trates a protocephalon levator in Astacus and 

 theoretically one would expect to find the muscle 

 in Pandalus. 



POSTERIOR 1'ROTOCEPHALON LEVATOR MUSCLES 

 Figures 5, 6, 34 



The function of moving the protocephalon dor- 

 sally is performed by a pair of large muscles, 

 the posterior protocephalon levator muscles, which 

 originate close together at the dorsal midline of 

 the carapace somewhat posterior to the origin of 

 the anterior protocephalon levator muscles and 

 which run forward and downward to attach on 

 a nearly vertical transverse plate, posterior to 

 the post ocular region of the eyestalk base (fig. 6). 

 The muscle inserts ventrally to the insertion of 

 the anterior levators. The contraction of the 

 posterior protocephalon levators may also act to 

 rotate posteriorly the eyestalk base and hence 

 raise the extended eyestalks. 



Possible homologs of the posterior proto- 

 cephalon levator muscles are the median dorsal 

 muscles designated as the musculus oculi basalis 

 posterior. In Astacus, Schmidt (1915) found that 

 these muscles arise on the median dorsal surface 

 of the carapace and are attached by short tendons 

 to the much longer tendons of other, more an- 

 teriorly placed muscles, the musculus oculi basalis 

 anterior. The anterior eye base muscles, to an- 

 glisize freely, are attached to the median dorsal 

 region of the eyestalk base (Schmidt 1915). 

 More will be said of the latter muscles below. 



The posterior eye base muscles, it should be 

 emphasized, do not attach to the eyestalk base in 

 Astacus, but if the assumption is made that, due 

 to the immovable protocephalon in Astacus, the 

 attachment of the muscles to the eyestalk base 

 has moved in that animal to the tendons of the 

 anterior eye base muscle, then a homology with 

 the posterior levators in the white shrimp may be 

 proposed. However, the extensive rearrangement 

 of muscle attachments upon which the assumption 

 is based weakens the proposal. 



Even more significant, muscles exist in Penaeus 

 setiferus, as will be shown later, which are more 

 likely to be the homologs of the anterior and 

 posterior eye base muscles in Astacus, Pandahi.s. 



and Callineetes than are the posterior proto- 

 cephalon levator muscles. If the latter is true 

 then the posterior protocephalon levators have 

 been lost during the evolution of Astacus, Pan- 

 dalus, and CaUincctes, in which forms no trace of 

 the muscles appears (Schmidt 1915; Berkeley 

 1928; Cochran 1935). 



OCULAR PLATE DEPRESSOR MUSCLES 



Figures 5, 6, 7 



The ocular plate depressor muscles originate on 

 th" posterior surface of the epistomal invagina- 

 tion. They run anterodorsally, passing beneath 

 the insertions of the posterior protocephalon 

 levator muscles, and insert broadly on the pos- 

 terior edge of the ocular plate (figs. 5, 6, 7). On 

 contraction, the ocular plate depressors draw the 

 ocular plate posteriorly and ventrad. Based on 

 the attachment points of the muscles in Penaeus 

 setiferus, they may have given rise by partial 

 fusion to the anterior eye base muscles (musculus 

 oculi basalis anterior) as they are found in the 

 European crawfish, where the muscles are attached 

 ventrally to the epistomal region by a long tendon 

 and run dorsad to the edge of the eyestalk base. 

 Cochran (1935) describes in Callineetes a pair of 

 anterior eye base muscles which arise from a kind 

 of epistomal invagination rather like that in the 

 white shrimp, but instead of fusing as in the Euro- 

 pean crawfish, they diverge slightly laterad in the 

 blue crab in probable accompaniment with the 

 general broadening of the body in the Brachyura. 



The ocular plate depressor muscles apparently 

 are homologous with the musculus oculi basalis 

 anterior in Pandalus. the name for which Berkeley 

 (1928) has taken from Schmidt (1915). In 

 Pandalus, these muscles are similar to those in 

 Penaeus. except that they are slightly separated, 

 whereas in P. setiferus they are close together. 



PROTOCEPHALON ATTRACTOR MUSCLES 



Figures 5, 6, 7, 8 



The protocephalon attractor muscles are two 

 very large muscles which take broad L-shaped 

 origins on the carapace and run anteriorly to in- 

 sert on two pairs of large apodemes and on other 

 parts of the protocephalon. The largest apodeme, 

 on which the ventral-most part of the proto- 

 cephalon attractor muscles inserts, arises from the 

 ventral surface of the antennular foramen, 

 broadening posteriorly into a large vertical sheet 



