NO. II THORACIC MUSCLES OF COCKROACHES — CHADVVICK 9 



to the length of 2sps-SiiA; and this step, to judge by various instances 

 observed, leads to an intervening stage in which the now floating 

 muscle is still divided by a transverse septum at the original level of 

 attachment on Sua. (figs. 3, 17: 21). Subsequently, all signs of the 

 septum are lost. Reduction of the ventral region of the first abdomi- 

 nal segment has doubtless contributed to developments of this nature, 

 which are not confined to cockroaches or to the particular muscle 

 in question (cf. fig. ii : ji*). 



2. THE INTERSEGMENTAL LATEROSTERNITES 



Intersegmental laterosternite (Us) is a term here introduced for 

 sites of muscle attachment that lie at the lateral extremities of the 

 ventral intersegmental folds. Such sites are believed to have been 

 characteristic features of the anatomy of early arthropods in all in- 

 tersegments. In existing forms, extensive modification of the original 

 relationships is the rule, as will be seen below ; nevertheless, recogni- 

 tion of the presence and nature of these sites is helpful in understand- 

 ing the manner in which the ventral musculature and associated 

 structures have evolved. 



Primitively, the musculature of the Us included (i) the transverse 

 muscles, which, with or without interruption by a median spina, 

 stretched between the two Us of the same intersegment; (2) the out- 

 ermost bands of the ventral longitudinal body musculature; (3) cer- 

 tain dorsoventral muscles ; and probably (4) other muscles of vari- 

 ous types, some of which will be noted below. However, the original 

 muscular relationships of the Us are still not fully understood ; and 

 this fact, together with their varied fate in different lines of descent 

 and in different parts of the body, presents the comparative morphol- 

 ogist with many perplexing problems. Hence, it is not surprising that 

 structures equivalent to the Us as here defined have been overlooked 

 by some workers and variously named in different situations by 

 others. Several have referred to them as "intersegmental pleurites," 

 a term which is unsatisfactory both because of the obvious sternal 

 nature of the structures in question and because use of the name 

 "pleurite" in reference to intersegmental elements is self-contradic- 

 tory. Crampton (1926) avoided these objections by employing the 

 term "furcilla." Unfortunately, this usage of "furcilla" seems likely 

 to cause confusion, because the name had been applied in various 

 other senses by earlier workers and because it suggests a nonexistent 

 affinity with the segmental sternal apophyses, or "furcae" (/«)• For 

 these reasons, we have substituted the more accurately descriptive 

 desisfnation "Us." 



