ON THE GENERAL MORPHOLOGY OF THE MYXINOID FISHES. 



745 



In Bdellostoma, according to J. Muller, the anterior extremity of the obliquus is 

 considerably narrowed down, and is not a blunt dorsal expansion as in Myxine. 

 Muller was the first to point out that there is a very striking difference between 

 Bdellostowxi and Myxine, since in the former the obliquus passes external to the slime 

 sacks, the apertures of which open between the muscle fibres. Here, too, the com- 

 pressing action of the obliquus affects the slime sacks, but of course in a different way. 



32. M. rectus. (Figs. 1, 3, 5, 9, 11, rect.). 



J. Muller, Gerade Bauchmuskel (p. 247). 



P. Furbringer and Wiedersheim, M. rectus abdominis (p. 89 and p. 233). 



M. Furbringer, M. rectus (pp. 614 and 628). Hypobranchiale Musculatur (p. 627). 



I have adopted Max Furbringer's term for this muscle, as being less misleading 

 than his brother's. The two recti are covered throughout by the obliqui, and for the 

 greater part of their course they lie roughly between the two rows of slime sacks (fig. l). 

 They arose in the specimen now described (26 cm.) 7 mm. in front of the cloaca by 

 elongated spindle-shaped muscular strands, connected by slender fibro-muscular cords 

 with the narrowed anterior end of the strong sphincter cloacae. These strands soon 

 shorten and thicken to form the paired series of (generally) alternating myomeres which 

 constitute the rectus muscle. Throughout the entire abdomen, however, the myomeres 

 are of a transversely wide irregular shape, are separated by extensive ligamenta inter- 

 muscularia (inscriptiones tendinege), and, as at the remainder of their course, are in 

 contact at the mid-ventral line. Abdominally, the rectus is in fact more or less 

 rudimentary, and forms the mid-ventral boundary of the body cavity. It narrows down 

 to pass between the branchial orifices, but in front of the latter at once begins to 

 gradually increase in width and strength, until in the present specimen it was widest at 

 about 11 mm. behind its anterior extremity. In front of its widest part it rapidly 

 flattens from side to side into a vertical ribbon, which creeps between the median and 

 lateral heads of the copulo-glossus profundus, to be inserted (usually) by a very short 

 ligament into the sheath of the posterior extremity of the middle segment of the basal 

 plate (near its junction with the posterior segment) at about the middle of its ventro- 

 lateral surface. 



The segmentation of the rectus may be tabularised as follows (26 cm. Hag) : — 





Right Side. 



Left Side. 



Myomeres of 

 Parietalis. 



Myomeres of 

 Rectus. 



Myomeres of 

 Rectus. 



Myomeres of 

 Parietalis. 



In front of Branchial Apertures .... 

 Opposite Branchial Apertures .... 

 Between Branchial Apertures and Cloaca . 



22 



1 



50 



19 



1 



52 



21 



1 



56 



22 



1 



55 



Total 



73 



72 



78 



78 



