734 MR FRANK J. COLE 



Internally, Both sides : — 



When the internal surface of the muscle is examined it is seen that there are a 

 number of fibres in addition to those appearing on the external surface. Ignoring the 

 scattered fibres running in various directions, and which may occur on the external 

 surface also, we notice, particularly in the mid-dorsal region, a large numbar of fibres 

 coursing longitudinally, which may be more or less diffuse, or which may be collected 

 together into a definite tract on each side of the mid-dorsal line. I shall call this the 

 dorsal longitudinal tract. It increases in thickness as it passes backwards. In front, 

 these fibres pass over into the anterior muscular limb already described (c. b. c'.). It is 

 now seen that the loops are not entirely formed by the dorso-ventral fibres of the outer 

 muscular sheet of the constrictor, but receive from, and give fibres to, the dorsal 

 longitudinal tract. The greater majority of the latter fibres pass from the tract 

 on to and along the posterior margins of the loops. From the anterior margins of the 

 loops a few fibres are given off which accompany the tract forwards. It would appear, 

 therefore, from this, that many fibres of the loops have no beginning and no end, but 

 course round in circles formed by the dorsal and ventral longitudinal tracts and the 

 loops. Behind, the dorsal tract behaves as follows : — 



1. A few of the fibres may pass straight backwards, ventral to the vascular gap 

 (figs. 2, 3, 13, 12), and along the inner surface of the cardiac portion of the 

 constrictor. 



2. Most of the fibres curve downwards in front of the vascular gap (12), and behind 

 the ductus oesophago-cutaneus on the left side and the last efferent gill duct on the 

 right, to meet at the mid-ventral line on the gut internal to the origin of the ventral 

 longitudinal tract, and thus continue forwards the cardiac portion of the constrictor. 



3. On the left side a number of fibres were contributed to the sixth and largest 

 loop (figs. 2, 3, 13, 6), which passes obliquely forwards in front of the ductus oesophago- 

 cutaneus, and which varies less than any other portion of the constrictor. 



4. A few fibres may bend forwards and go over into the external portion of the 

 ventral insertion (fig. 13, 13). 



Sixth Loop (6). — As this has already been described on the right side, the present 

 account applies to the left side only. It arises from the inner surface of the outer 

 branchial muscular sheet, and passes downwards and forwards into the outer portion of 

 the ventral insertion in front of and external to the ductus cesophago-cutaneus, and 

 internal to the last efferent gill duct. It is constituted as follows : — 



1. Some dorso-ventral fibres from the internal surface of the muscular sheet above. 



2. A number of fibres from the dorsal longitudinal tract, as above described. 



3. The fibres forming the definite constrictor of the ductus oesophago-cutaneus 

 (figs. 2, 3, 13, 14), which accompany the sixth loop in front of the ductus, but leave it 

 above and below to complete the sphincter by curving round the back of the ductus. 



For the insertion of this loop, see below. 



That the sixth loop is serially homologous with the preceding five is borne out by 



