272 HOVEY JORDAN 



but becomes so changed in the course of phylogenetic development as 

 to function solely as a temperature terminal. 



So far as the hamlet is concerned, it should be noted that the integu- 

 ment of its head is sensitive to a temperature change of about 11C. 

 (from 21 to 10), because water at 10,C. causes a very precise negative 

 reaction. Since its integument contains receptors for both photic and 

 thermal stimulation, it presents a case in which, according to the view 

 of a primitive and more or less universal radiant-energy receptor, the 

 two functions may inhere in the same nerve terminal; but it should be 

 emphasized that the other view -original independence of heat and 

 light receptors is likewise defensible. It is, moreover, of interest in 

 connection with the above theory of a specialization of function, that 

 in some of the vertebrates above fishes the integumentary photosensi- 

 tivity is not entirely lacking. To what extent it persists is as yet 

 known only imperfectly. Graber ('84), Dubois ('90), Koranyi ('95) 

 and Parker ('03) have shown that certain Amphibians possess it. Some 

 reptiles also, according to Carlton ('03) and others have skins which are 

 sensitive to light. It is noticeable, too, that as between fishes and 

 amphibians this photosensitivity is negative in the former, but positive 

 in the latter (Parker '03) . 



More direct evidence of degeneration is seen in Amblyopsis, where, 

 according to Payne ('07) the cutaneous photosensitivity is more pro- 

 nounced in the young than in the adult. This fish, however, having 

 functional eyes in the younger stages but lacking them in the adult, 

 has probably been independently specialized for a peculiar mode of 

 life and presents an unusual simultaneous degeneration of both optic and 

 cutaneous photosensitivity, rather than ontogenetic evidence parallel 

 to the assumed phylogenetic degeneration. 



Parker ('08), after having studied the direction eyes of Amphioxus, 

 concluded that Boveri ('04) was right in claiming that they arise in 

 situ, and-since the uniform orientation required by Balfour's theory does 

 not in his opinion exist in Amphioxus, he considers the two forms of 

 sensitivity (cutaneous and optic) to be independent of each other. 

 This reasoning presupposes, of course, that the direction eyes of Am- 

 phioxus are the more or less immediate predecessors of the rods and 

 cones. At first sight the promiscuous arrangement of the functional 

 eye-cups in Amphioxus does seem to be an objection of Balfour's 

 theory; but it is conceivable that these eye-cups may be secondarily 

 oriented in different directions because light enters the cord from all 

 directions, but that this interference with the uniform orientation of 



