No. 2.] MORPHOLOGY OF THE MYXINOIDEI. 21 3 



Moreover, the relations which exist on the opposite sides, 

 even of the same animal, are quite different in all the specimens 

 examined. The heterogeneity of segmental arrangements in 

 myxinoids, of which this is an example, is one of the most 

 remarkable characteristics of their anatomy. 



The last external gill passage on the left side always, so far 

 as our observations yet go, opens into the oesophago-cutaneous 

 duct, instead of having an independent opening on the sur- 

 face of the body. In the wall of the oesophago-cutaneous 

 duct is a cartilage of peculiar shape (PI. XXII, Fig. 6, oes. c. ; 

 PL XXIII, Fig. 13). This is the only part of the gill-cartilage 

 system heretofore observed in Bdellostoma. 



It is irregular in shape and varies considerably in different 

 specimens. Apparently it has been derived by the fusion of 

 two cartilages such as we find in the other gill tubes ; one 

 corresponding to the last left gill tube, and the other to the 

 oesophago-cutaneous duct. It is an important fact that one 

 process from this cartilage (PI. XXIII, Fig. 13, h) often passes 

 up the duct wall and into the luall of the oesophagus, i.e., 

 pharynx. 



The number of gill cartilages (including that of the oesoph- 

 ago-cutaneous duct), of course, corresponds to the number of 

 gills. This number, as Dr. Ayers has pointed out, varies to a 

 remarkable degree. In 354 specimens of Bdellostoma dombeyi 

 collected by him from Monterey Bay in the spring of 1892, 

 the following variations occurred : 



loi individuals had 11 gills on both sides. 



354 



Of the eleven to twelve variation, where the position of the 

 gills was noted, four had eleven gills on the right side and 

 twelve on the left; while four were just the reverse, with 

 twelve gills on the right side and eleven on the left. The 



