746 MR FRANK J. COLE 



There is thus a close agreement in total number between the two segmented muscles 

 even when the numbers of the two sides do not agree, and this agreement also extends 

 in general to the ligamenta intermuscularia. Therefore, as the myomeres of the rectus 

 are generally opposite those of the parietalis, the former may conceivably represent the 

 disassociated ventral portions of the latter. Hence the myomeres of the parietalis 

 should (and do) alternate. These statements, however, do not apply to the whole 

 length of the rectus in every specimen, as the two sides may in parts agree or alternate 

 with the parietalis in the same individual. The latter condition is shown in fig. 5. 



These discrepancies are to be expected, since the state of contraction of the muscle 

 will necessarily affect its position. J. Muller also favours the view of the complete 

 correspondence of the myomeres of the rectus and parietalis, and states further that 

 these two muscles are completely interrupted by the row of slime sacks in Bdellostoma. 

 The latter cannot be said to apply to Myxine (fig. 1) ; and if it did, its significance would 

 be neutralised by the fact that the presence of the slime sacks has not affected the 

 obliquus, lying between the rectus + parietalis and the slime sacks. Further, according 

 to J. Muller, the myomeres of the two recti are symmetrical ; and as the myotomes of 

 Bdellostoma alternate, according to Ayers and Jackson (although this is not as easy to 

 determine as one would imagine), this should not be the case. Wiedersheim correctly 

 states that the myomeres of the rectus alternate in Myxine, and assumes that this 

 muscle represents a detached portion of the parietalis. M. Furbringer seems to 

 favour the same view, but draws attention to a certain asymmetry between the two 

 muscles, i.e. there are fewer hypobranchial (rectus) than epibranchial (parietalis) 

 myomeres. This I also find. The whole question, however, has been reopened 

 by the work of Wikstrom, # and must therefore be left over until the nerves are 

 considered. 



The rectus clearly performs a double function. In the abdomen it assists in flexing 

 the body ventrally, but owing to its feeble development at this region its action 

 cannot be powerful, whilst its stronger anterior portion assists in the retraction 

 of the basal plate, that is, when the extension of the body is preserved by the 

 parietalis. 



Ammoccetes and Petromyzon also have an anterior representative of the rectus, as 

 first pointed out by A. Schneider, f 



Since writing the above I have dissected a 45^ cm. Hag in which the rectus was 

 inserted, not into the basal plate, but into the external fascia of the copulo-glossus 

 profundus. The two heads of the latter muscle diverged as usual to admit the passage 

 of the rectus, but it terminated abruptly on both sides 14 mm. behind this place. This 

 very interesting variation seems to emphasise the vestigial (or rudimentary ?) nature of 

 this muscle. 



* Anat. Ami., xiii. p. 401, 1897. He concludes from the innervation that the two muscles are not homologous, 

 segment for segment. 



t Zool. Am., v. p. 164, 1882. 



