2O 



JEAN DAWSON. 



The other -half-ring shaped muscle (cntal muscle] is smaller 

 and extends loosely around the caudal side of the gill sac near 

 its external opening but between the gill sac and its pouch. 

 Thus it cannot be seen unless the muscular pouch be cut and 

 reflected. It is attached to the same longitudinal rods of carti- 

 lage as is the ectal muscle, a little beneath the points of attach- 

 ment of the latter and is attached slightly to the ring of cartilage 

 (Fig. 8, ) which sends out a flat semicircular projection. This 



projection lies on the caudal 

 side of the external gill open- 

 ing within the ring of carti- 

 lage. It bears a slender con- 

 ical rod of cartilage (Figs. 10 

 and 12, e). 



The ectal and ental mus- 

 cles are striated and function 

 in compressing the sides of 

 the ring of cartilage so that 

 the naturally circular cartilage 

 becomes elliptical with its long 

 axis dorso ventrad. Cuvier 

 (1840) and Mayer (1835) 

 The ectal muscle and its at- speak of a circular muscle of 



/, cartilaginous bars ; />, cartilag- the gill Opening which lies 



around the ring of cartilage 

 and serves to close the gill opening. Just how a constrictor 

 muscle, if such a muscle were present, could so compress the car- 

 tilaginous ring as to completely close the opening is not clear. 



A fringe of hair-like processes lying around the caudal border 

 of the external gill opening deserves mention. Cephalad about 

 I mm. from the middle of this fringe is the small cartilaginous 

 projection which is the outgrowth of the cartilaginous ring be- 

 fore mentioned. The function of these outgrowths is not known. 

 They may be tactile. 



D. Nasal Sac. - - The nostril is situated on the dorsal side of 

 the head in the median line about 5 mm. anterior to the paired 

 eyes. This nostril leads into a tube which opens directly into 

 the nasal sac with its olfactory lamellae. From the cranial por- 



FIG. 14 

 tachment. 

 inous ring ; <, ectal muscle. 



