NO. 6 GRASSHOFTER ABDOMEN SNODGRASS 33 



diaphragm membrane appears to be everywhere continuous with a 

 deHcate peritoneal covering over the inner surfaces of the somatic 

 muscles lying lateral of the pericardial cavity. The upper membrane 

 of the diaphragm, however, being reflected upon the dorsal pericardial 

 wall, more clearly marks the limits of the diaphragm itself. The two 

 membranes of the diaphragm can be distinguished in whole prepara- 

 tions under the microscope by the two layers of nuclei, one dorsal to 

 the muscle fibers, the other ventral. It is apparent that the two mem- 

 branes, however, are simply continuations of a peritoneal lining of the 

 perivisceral cavity and of a similar lining of the pericardial cavity, 

 with the transverse muscles between them. 



The muscles of the dorsal diaphragm in Dissostcira begin in the 

 second segment of the abdomen (fig. 15 A) and end in the ninth 

 segment (B). They consist of a double series of transverse fibers, 

 separated into segmental groups, but for the most part approximately 

 parallel. In all but the second and ninth segments the fibers are slightly 

 divided into secondary anterior and posterior groups. This intra- 

 segmental segregation of the fibers is more accentuated in Mclanoplus 

 than in Dissosteira. The fibers arise laterally on the tergal plates be- 

 tween the median and the lateral longitudinal dorsal muscles (figs. 

 8, 10) . Their median ends branch toward the ventral wall of the heart, 

 on which they break up into fine fibrils, and the fibrils from opposite 

 sides appear to unite in an intricate plexus. 



The dorsal blood vessel extends from beneath the brain into the 

 tenth abdominal segment. Ostia and slight segmental enlargements of 

 the tube are present in abdominal segments // to IX (fig. 15 A, B). 

 Dorsal ampullar enlargements of the aorta occur in the mesothorax, 

 metathorax, and first abdominal segment. Posteriorly the heart term- 

 inates in a narrow tapering tube extending into the tenth abdominal 

 segment. For most of its length the dorsal vessel is accompanied by 

 strands of nephrocytes. and the aortic ampullae are capped by dense 

 masses of nephrocytic cells. The pericardial cavity contains also loosely 

 scattered fat cells, and is penetrated by loops of the Malpighian tubules. 

 The dorsal longitudinal tracheal trunks (fig. 15 A, B) lie along the 

 sides of the blood vessel and are connected with the lateral trunks by 

 transverse tracheae in the posterior part of each abdominal segment. 

 It would appear that the blood has entrance into the pericardial cavity 

 only above the free lateral margins of the diaphragm in the first ab- 

 dominal segment, and at the posterior end of the diaphragm in the 

 eighth and ninth segments. 



The ventral diaphragm in Dissosteira extends from the head into 

 the seventh (female) or eighth (male) abdominal segment. In the 



