ON THE GENERAL MORPHOLOGY OF THE MYXINOID FISHES. 739 



of the insertion. All these form a large paired tract which passes backwards internal to 

 the gill and oesophageal ducts, and lateral to the cardiac aorta. As these tracts course 

 posteriorly they spread out dorso-ventrally, and finally, together with the two recti 

 below, by fusing at about the middle line under the gut, they form a complete muscular 

 tube enclosing behind the vascular sinus surrounding the cardiac aorta. The mass then 

 fuses with the posterior external sheet of the constrictor to form the cardiac portion of 

 the muscle in such a way that fibres from each tract curve round both sides of the gut. 

 There is thus a well-marked decussation, as mentioned above. 



30. M. parietalis, F.J.C. (Figs. 1, 4, 5, 7, 8, 9, 14, par.) 



J. Muller, Riicken- und Seitenmuskeln (p. 243). 

 M. Furbringer, Epibranchiale musculatur (p. 627). 



This is the muscle that constitutes the well-known alternating my tomes of the fish. 

 Its division into dorsal and ventral portions (Parietalis dorsalis and Parietalis ventralis) 

 by a horizontal septum stretching from the vertebral column to the lateral line, which 

 obtains in other fishes, does not exist in Myxine, where this septum is wanting. 

 Wikstrom,* however, considers the septum a secondary formation of little morphological 

 importance, and J. Muller distinguishes " back " and " lateral " muscles in the parietalis 

 of Bdellostoma, although this division is certainly artificial, as M. Furbringer states. 

 Wiedershiem considers the obliquus + rectus as representing the detached ventral 

 musculature, and actually labels the dorsal border of the obliquus as the linea lateralis. 

 Omitting the question of the morphology of the rectus, the parietalis of Myxine is in 

 transverse section a single sickle-shaped muscle on each side (fig. 1), the two being 

 separated mid-dorsally by a vertical partition arising from the cranium and neural tube, 

 and passing over into the strong external fascia of the muscle. This septum is in front, 

 over the cranium, wide from side to side and narrow behind, as shown in fig. 8. The 

 parietalis terminates some distance from the mid -ventral line by a free border situated 

 just above the slime sacks (fig. 1). The same conditions apply to Petromyzon for the 

 region behind the gills, but within the branchial area dorsal and ventral parietal muscles 

 have been differentiated. 



In Fishes generally the parietalis does not extend further forward than the occipital 

 region of the skull, but in Myxine it passes forwards over the membranous cranium until 

 it almost reaches the posterior boundary of the nasal capsule (see figs. 8 and 9). 



The parietalis, enclosed externally and internally by a stout fascia, the fibres of which 

 in the outer one course obliquely downwards and forwards, is subdivided by the zigzag 

 vertical partitions or septa intermuscularia t (ligamenta intermuscularia, J. Muller ; 

 inscriptiones tendinese) into a number of muscle segments or myotomes. These septa 



* Anat. Anz., xiii., 1897, p. 401. 



1 1 have avoided the term myocomma for the septum, as this term was used by the older anatomists to indicate 

 the myotome itself. 



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