140 



BIOLOGICAL LECTURES. 



8th pairs of gills, according to Putnam. In the 12 and 13-gilled 

 forms I have found it between the 5th, or at most, the 6th pairs 

 of gill-sacks. 



In the material which I was able to collect at Monterey the 

 following proportions of the several variations prevailed : 



[oi individuals liad ii gills on both sides. 



26 " "II " " one side 



and 12 " " the other side. 



208 " had \i " " both sides. 



II " " 12 " " one side 



and 13 " " the other side. 



8 " had I ^ " " both sides. 



354 total number of individuals counted. 



Fig. 7. — The olfactory organ 

 seen from its ventral surface. ^A', 

 nasal tube. /-, the nasal plates, 

 of which there are three on either 

 side of the median raphe. K. 

 X 2 diameters. 



Fig. S. — A transverse section of the 

 olfactory organ of Bdellostoma, showing 

 the nasal plates cut across G, and also 

 partly in surface view, S. R, the raphe, 

 and Z, the lateral wall of the nasal cap- 

 sule. X 4 diameters. 



Of the eight II -1 2 variation, where the position of the gills 

 was noted, four had 1 1 gills on the right side and 1 2 on the 

 left, while the other four were just the reverse, with 12 gills 

 on the right side and i i on the left. The same alternate vari- 

 ation holds true of the 12-13 variation. This fact of alternate 

 variation proves conclusively that the variability in number of 

 gills between the two sides is in no way connected with the 

 formation of the ductus oisophago-cutaneus. which is always 

 upon the left side. 



The vascular svstem of Iklellostoma is being studied in 

 Prof. Gilbert's laboratory at Stanford University, and I need 

 not enter into the interesting relations which this system ot 

 organs sustains to the general variability of this animal, 



