34 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 85 



it occurs on the region of the dorsum, being situated above the Hne of 

 the spiracles, and therefore not on the true pleural region, the writer 

 would term it the paratergal muscle (p). Since this muscle belongs 

 to the dorsum it should be classed as a dorsal muscle. 



The fibers of the transverse muscles are never differentiated into dis- 

 tinct layers, but they may be variously grouped in both the dorsal 

 system (fig. 13 C, td) and the ventral (tv). 



The spiracular muscles comprise usually not more than two mus- 

 cles associated with each spiracle. One is an occlusor of the spiracle, 

 the other a dilator. The occlusor is seldom lacking ; the dilator is less 

 constant. 



To express more concisely the principal groups of abdominal mus- 

 cles and their subdivisions, we may tabulate the muscles in the follow- 

 ing manner : 



I. MUSCULI DORSALES (d) . 



1. M. dorsales interni (di). 



a. M. dorsales interni mediales (dim). 



b. M. dorsales interni laterales (dil). 



2. M. dorsales externi (de). 



a. M. dorsales externi mediales {dern). 



b. M. dorsales externi laterales {del). 



3. M. paratergales {p). 



II. MUSCULI VENTRALES (^'). 



1. M. ventrales interni {vi) . 



a. M. ventrales interni mediales {vim). 



b. M. ventrales interni laterales {vil). 



2. M. ventrales externi {ve). 



a. M. ventrales externi mediales {vem). 



b. M. ventrales externi laterales {vel). 



JIT. MUSCULI LATERALES (/). 



1. M. laterales interni {li). 



2. M. laterales externi {le). 



IV. MUSCULI TRANSVERSALES (/). 



1. M. transversales dorsales {td). 



2. M. transversales ventrales {tv). 



V. MUSCULI SPIRACULORUM {s) . 



1. M. occlusores spiraculorum {os). 



2. M. dilatores spiraculorum {dls). 



Each secondary group of muscles is often again subdivided into 

 seA^eral bundles of fibers. These ultimate individual muscles may be 

 given numerical designations, beginning medially in the case of the 

 longitudinal muscles and anteriorly with the lateral muscles. Thus 



