132 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



sertion on the ventromedial edge of the protopo- 

 dite. The muscle turns the endopodite toward 

 the midline, with the result that the endopodites 

 are closed. The endopodite adductor of Penaeus 

 is probably homologous with the same muscle in 

 the tail fan of Pandalus and Astacus. 



ANTERIOR TELSON FLEXOR MUSCLE 



Figures 70, 72, 73, 74 



The anterior telson flexor muscle, originates over 

 most of the anterodorsal portion of the telson (fig. 

 70). The muscle passes directly ventrad to insert 

 on the apodeme shared by the dorsal rotators and 

 the telson flexors. The muscle has almost exact 

 counterparts in the telson of Pandalus and 

 Astacus. The anterior telson flexor muscle pulls 

 the telson downward upon the uropods in the 

 vertical plane. 



POSTERIOR TELSON FLEXOR MUSCLE 



Figures 70, 71, 72, 73, 74 



The posterior telson flexor muscle is much 

 larger than the anterior telson flexor. The pos- 

 terior telson flexor originates over all of the dor- 

 sal and lateral tergum of the telson not taken up 

 by the small anterior flexor. The muscle fibers 

 run anteroventrally, narrowing to their insertion 

 on the common apodemal material to which the 

 dorsal rotators and anterior telson flexor are at- 

 tached (fig. 74, A). The posterior flexor rein- 

 forces the action of the anterior telson flexor in 

 bringing the telson ventrad. The posterior telson 

 flexor muscle of Penaeus is represented in similar 

 form in Astacus and Pandalus. 



VENTRAL TELSOX FLEXOR MUSCLE 



Figure 74, B 



The ventral telson flexor muscle is located 

 within the sixth abdominal segment dorsal to the 

 anterior oblique muscle apodemes. The muscle 

 occupies a scoop-shaped depression in the postero- 

 dorsal part of anterior oblique muscle 6 (fig. 74, 

 B) where it takes its origin. The muscle passes 

 directly caudad to insert on an apodeme arising 

 from the anterior margin of the telson sternum. 

 The muscle turns the telson ventrad in the vertical 

 plane. A counterpart of the ventral telson flexor 

 muscle of Penaeus is found in Pandalus. The 

 muscle is missing in Astacus. 



ANAL COMPRESSOR MUSCLES 



Figures 71, 72 



The anal compressor muscles lie ventral to the 

 rectum and alongside of it and insert into the 

 lateral side of the anal opening. Contractions of 

 the muscles flatten the sides of the anal opening 

 in the long axis and thereby tend to close it. A 

 similar muscle is found in Astacus. but not, ac- 

 cording to Berkeley, in Pandalus. 



ANAL DILATATOR MUSCLE 

 Figure 72 



The anal dilatator muscle runs at right angles 

 to the anal compressor and slightly above it (fig. 

 '■2, A), connecting into the lateral tergum of the 

 telson by a tendon. Contractions of the dilatator 

 open the anal aperture. The same muscle is found 

 in the other crustaceans to which we have made 

 reference. 



RECTAL ATTRACTOR MUSCLES 

 Figure 74, B 



The rectal attractor muscles are two small mus- 

 cles of doubtful affinity not described by Schmidt 

 in Astacus or Berkeley in Pandalus. The muscles 

 lie in the midline of the sixth abdominal segment, 

 between the uropod rotators. Both are attached 

 to the median dorsal edge of the telson. The 

 small, dorsal attractor passes rostrad to a tendinous 

 fascia associated with the uropod rotators. If 

 this muscle were larger it might better be named a 

 telson extensor, but its size argues against this 

 interpretation. The larger, ventral attractor mus- 

 cle inserts in connective tissue on the dorsal sur- 

 face of the rectum ami undoubtedly draws the 

 rectum caudad in movements associated with 

 defecation. 



II. THE NERVOUS SYSTEM 



To the comparative morphologist, the nervous 

 system is of fundamental importance. Of all the 

 systems — skeletal, muscle, alimentary, reproduc- 

 tive, etc. — the annulate nervous system has most 

 nearly retained its generalized form during the 

 evolution of the group. That is, the nervous sys- 

 tems of any two annulates are remarkably similar. 

 Nervous systems of Annulata, then, as has been 

 amply shown by Snodgrass (1938), Ferris (1953), 

 and others, are singularly conservative of form in 



