22 ARTHUR DENDY. 



over tlie plug, except sncli as I have to mention shortly in 

 regard to the pigment. Above the coi-ium comes the epi- 

 dermis, which again exhibits a perfectly normal structure. 



AVhen the dorsal sni-face of the head is examined carefully, 

 a small, light-coloured patch is visible a short distance behind 

 the nostril. This patch is somewhat elongated and nearly 

 oval in outline (perhaps, rather, key-hole shaped), and con- 

 stitutes the well-known '^ Scheitelfleck" of German authors. 

 It lies immediately above the parietal plug, and owes its pale 

 colour to the fact that the pigment, elsewhere so abundantly 

 developed in the integument, is here almost entirely absent. 

 Elsewhere we find the pigment cells arranged in two layers, 

 an outer and an inner. The outer one (fig. 2, Pig}) lies in 

 the corium at a very short distance beneath the epidermis, 

 and is but feebly developed, consisting of a sparse layer 

 of much -branched cells. This layer is still more feebly 

 developed in the region of the " Scheitelfleck," but not 

 entirely absent. The inner layer of pigment (fig. 2, Pig.") 

 lies immediately beneath the corium, which it separates from 

 the underlying looser connective tissue. It is very much 

 denser than the outer layer, and is completely absent beneath 

 the " Scheitelfleck," terminating abruptly on reaching the 

 upper margin of the parietal plug. 



From the foregoing account it will be evident that the 

 light-transmitting tissues which overlie the parietal organs 

 have essentially the same structure and arrangement as in 

 Petromyzon, but, if we ma}^ ji^*lge from Studnicka's figures, 

 the parietal plug is better defined in Geotria. 



(g) General Considerations. 



The function of the parietal sense organs. — In 

 considering the question of function, one must distinguish 

 sharply between the right and the left parietal sense organs. 

 The former is a well-developed "pineal eye," containing the 

 essential structural elements which one is accustomed to 

 associate with a light-perceiving organ, and, in common with 



