130 BIOLOGICAL LECTURES. 



is white. In other words, as age increases, those parts of the 

 body which come into frequent contact with objects other 

 than water lose their purple color. The color grows gradually 

 darker from the ventral surface to the dorsum. The skin is 

 attached to the surface of the body of the animal by a thin 

 median dor<pl ligament and by a broad median ventral area of 

 trabeculous processes. The gill holes show a narrow, whitish 

 border, which is especially well marked about the ductus 

 cesophago-cutaneus, or the tube which permits the water to 

 escape from the oesophagus without going through the gills 

 (Fig. 9, D). The cloacal opening is edged with white. Over 

 the region of the eyes the skin is pigmentless and transparent, 

 so that light penetrates freely to the eye below ; but this 

 corneal membrane is not in any way invaginated or fixed to 

 the surface of the eye, and, since the skin in this region is 

 very movable, the transparent area is very much enlarged 

 when compared with the size of the eyes (Figs. I and 5). The 

 latter are small and devoid of pigment in the retina, as far as 

 can be discerned from the exterior, and the whole bulb is 

 imbedded in a fat pad between the two diverging cutaneous 

 branches of the V nerve where they issue from the sides of the 

 head. The fish are very sensitive to light, and seek for cov- 

 ering until the head is shaded, when they come to rest, it may 

 be, with most of the body exposed. 



Bdellostoma is not provided with appendages, i.e., pectoral 

 or pelvic fins, and depends for locomotion upon movements of 

 its body, eel-fashion. It is a graceful and rapid swimmer. In 

 the aquaria it remains most of the time quiet upon the bottom, 

 resting upon its ventral edge or side. In order to maintain an 

 upright position, i.e., back uppermost, it is necessary for it to 

 curve the body more or less, in order to provide a base of 

 support. Oftentimes the curve is slight, as is always the case 

 when the animal is in poor health. The normal position is 

 that of a right or left-handed coil ; and they coil both ways, 

 apparently to rest the muscles occasionally (Fig. 3, a to //). 

 Sometimes the whole body is taken up in the formation of 

 the coil ; again, often only the posterior part is used, thus the 

 head-end being reserved free, and held straight or bent into an 



