122 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 37 



sented by a series of transverse or oblique bands whose attachments 

 on the body wall are commonly intersegmental. Some writers have 

 regarded such muscles as offshoots of the outer circular layer, though 

 other views are also tenable. Certain it is that the spinasternal muscles 

 are related in part to this third category, whatever its source, for they 

 share with it not only position in relation to attachment sites but the 

 equally distinguishing feature of passing across the body internal to 

 the principal nerve trunks. However, the definitive spinasternal mus- 

 culature is too diverse to be derived solely from such an origin, and 

 yet many of its components do not fit easily into either of the other 

 recognized categories. 



Posteriorly, the spinasternal muscles appear to be continuous with 

 the ventral diaphragm, a fenestrated muscular and membraneous 

 partition whose attachments on the anterolateral regions of the ab- 

 dominal sterna are related morphologically to those of the thoracic 

 transverse intersegmental muscles, but in some insects the ventral dia- 

 phragm itself extends into the thorax (Czihak, 1956), with attach- 

 ments on what I have called the intersegmental laterosternites (Us) 

 (Chad wick, 1957). 



According to Roonwal (1937), however, the ventral diaphragm is 

 formed in the embryo of Locusta from segmental mesoderm and is 

 topographically dorsal to the transverse intersegmental muscles, which 

 arise from the blood cell lamellae. Nevertheless, the muscular attach- 

 ments of the ventral diaphragm to the "third spina" of some cock- 

 roaches and other ptcrygote insects are ventral to those of the other 

 spinasternal muscles, whereas the transverse bands, where they occur, 

 are usually the most dorsal of all elements inserted on the spina (see 

 figures in Chad wick, 1957). Such apparent inconsistencies emphasize 

 the need for further careful study of the genesis of the several muscu- 

 lar categories and of specific definitive muscles in a variety of insects 

 and related forms. 



The midventral attachments of the transverse muscles of insects on 

 the interneural spinae may be secondary, although Wells (1944) has 

 described in an annelid, Arenkola, transverse (oblique) muscles that 

 originate dorsolaterally from the outer circular layer and form ventral 

 connections with the connective tissue sheath of the nerve cord. In 

 other worms and arthropods, the transverse muscles, when present, 

 often pass from one side to the other without interruption, or are 

 inserted centrally into a complex endoskeletal plate that is suspended 

 in the body cavity between the nerve cords and the gut. The "oblique 

 dorsoventral muscles" of Onychophora resemble those of Arenkola 



