ON THE GENERAL MORPHOLOGY OF THE MYXINOID FISHES. 733 



the constrictor is covered by the posterior extremities of the ventral longitudinal tracts, 

 which partly arise here near or at the mid- ventral line, and between or just behind 

 the external branchial apertures. 



Anteriorly the cardiac portion of the constrictor passes imperceptibly into the 

 branchial portion (c. b. a), so that no distinction between the two portions is possible. 

 The fibres now begin to course in various directions, but the following general 

 tendencies may be distinguished. 



Externally, Left side : — 



1. A small slip may be given off to the internal fascia of the obliquus (fig. 13, 9). 



2. A number of fibres bent round ventrally and anteriorly to complete the formation 

 of the ventral longitudinal tract (for remainder, see above). 



3. A large number of fibres course downwards and forwards round the postero- 

 internal surface of the ductus oesophago-cutaneus (figs 2, 3, 10) to partly form the 

 muscular tube through which the ductus passes, and finally to assist in forming the 

 large external portion of the ventral insertion (figs. 2, 3, 13, 8'). 



4. A small slip (figs. 2, 13, 11) passes downwards and slightly backwards on the 

 posterior surface of the ductus oesophago-cutaneus, to be mostly inserted into the 

 ventral posterior process of the cartilage of the ductus (fig. 13, x s .) 



5. We now come to the region where the fibres terminate below in a ventral border, 

 which may be attached to the body-wall by a ligamentous sheet, as above described. 

 Here the fibres nearest this ventral border, after previously coursing downwards and 

 slightly backwards, turn forwards at a right angle and travel parallel to the ventral 

 border (fig. 13). A few fibres, however, here pursue a rectangular course (fig. 13, part 

 of 11), i.e. they pass first of all posteriorly parallel to the ventral border, and then, near 

 the posterior surface of the ductus oesophago-cutaneus, they turn down at a right angle 

 to be inserted as the slip (4) just described, and also into the wall of the ductus near its 

 external opening. As we go further forwards the fibres of the posterior region of the 

 branchial portion of the constrictor bend at a gradually increasing angle until finally 

 attaining the downward-forward direction characteristic of the greater part of the 

 muscle, as already described. 



Externally, Right side : — 

 1 and 2. As on left side. 



3. We find similar fibres coursing downwards and forwards, but this time round the 

 postero-internal surface of the last efferent gill duct, to form the external portion of 

 the ventral insertion. As these fibres course morphologically in front of the ductus 

 oesophago-cutaneus, they cannot of course represent the fibres of paragraph 3 of the 

 left side. They correspond in fact to the larger sixth loop of the left side, to be 

 described below, and which passes between the last efferent gill duct and the ductus 

 (figs. 2, 3, 13, 6). 



4. Absent. 5. As on left side. 



