446 CYCLOSTOMATA 



velopment within the agg, is slow compared with that of many 

 other vertebrates. 



One of the many interesting points about Myxinoids is 

 the variability exhibited by Bdellostoma as regards the number 

 of gills. A short account of this will serve to show how the 

 study of variations bears on the study of species. The Cali- 

 fornian hag has usually eleven or twelve gill-slits ; it was 

 called B. stouti. The Chilian hag with ten, was called B. dom- 

 beyi. The Cape of Good Hope hag, with six or seven, was called 

 B.forsteri. Ayers studied these three species and came to the 

 conclusion that they were really variations within one species 

 which is best called B. dombeyi. A Japanese form which Bash- 

 ford Dean called Homea okinoseana has eight gills, thus filling 

 the gap between the Cape form and the Chilian form. 



Miss Julia Worthington, combining the records of Dr. Gil- 

 bert and Dr. Ayers with her own, gives the following striking 

 table :— 



958 



Twelve on each side is the number most commonly found in 

 the Californian hag, but as it only occurs in about 60 per cent 

 in a total of 958, it may be said that eleven and twelve are the 

 usual numbers. It is very striking to find that 137 per cent 

 (out of 958) have more gills on one side than on the other. In 

 this connection Miss Worthington observes: "With six and 

 seven gills the prevailing number in Bdellostoma foi'steri, eight 

 in that found in Japan, ten the number in the Chilian form, 

 and eleven and twelve in California, varying on the one hand, 

 though rarely, to ten, and on the other hand, more frequently, 

 to thirteen, it is surely no longer possible to divide these animals 

 into different genera and species on the basis of the number of 

 gills alone; the count of teeth (Ayers, 1894), is equally unsatis- 

 factory as a ground for division into species, and no other 

 ground for such division has ever been advanced." It should 



