Dec, 1903.] Snodgrass : Anatomv of Pkranabrls Scabricoi.lis. 185 



The two dorsal longitudinal tracheal trunks lie just above the edges 

 of the diaphragm (Fig. 12, d. I. fr.). The paired transverse commis- 

 sures (/. /r. ) join the longitudinal trunks at the middle of each seg- 

 ment from the first to the ninth. In the tenth segment there is only 

 one transverse trachea on each side. 



The diaphragm muscles (Fig. 12, dp. m.) are fan-shaped bundles 

 of fibers that diverge upon the diaphragm from the anterior edges of 

 the terga between each two pairs of transverse trachese. After enter- 

 ing upon the diaphragm the muscles break up into minute fibrillne 

 (Fig. 9) that cross and are interwoven in all directions, so that one 

 individual fiber cannot be followed. In many cases the muscle appears 

 to end after breaking up into fine hair-like bunches of spreading 

 fibrillje (Fig. 10). 



The diaphragm is an almost invisible membrane but its presence 

 is indicated by its numerous nuclei (Figs. 9 and 10, d. n.). The 

 membrane itself, however, can be seen between the muscle fibers 01 

 the fan-shaped bundles. Mesially the diaphragm is imperforate, but 

 laterally it is fenestrated by numerous large and small oval holes (Fig. 

 9). In Dissosteira it is everywhere imperforate and is, in this form, 

 much more plainly visible than in Peranabriis because the muscle 

 fibers on it are not so numerous and do not break up into a felt-work 

 of fibrillar. Fig. 10 is a detailed copy of a small piece of the per- 

 forated part of the diaphragm. Several fenestra are shown with the 

 muscle bundles splitting and going around them as diverging groups 

 of fibrillrc. The latter are seen in several places disappearing by 

 becoming lost in a felt-work of fibers. The diajjhragm nuclei (^/. ;/. ) 

 are seen irregularly scattered about. In Dissosteira the muscle fibers 

 extend continuously across the diaphragm from one side to the other, 

 or branch and unite mesially in only a very simple plexus. They are 

 further in Dissosteira, not distributed upon the diaphragm in fan- 

 shaped bundles but arise serially or in small groups along the edge of 

 the diai)hragm and extend mostly straight across its surface. 



In Peranabriis the muscle fibers appear to be unstriated. In Dis- 

 sosteira they are distinctly striated. In the crayfish {Astaciis) the 

 floor of the pericardium is composed of an upper and a lower nucleated 

 membrane inclosing a layer of transverse striated muscle fibers. 

 Mesially the fibers break up into unstriated fibrillar but these can be 

 traced across directly into the fibers of the opposite side. 



The upper surface of the diaphragm is covered by a layer of small, 



