12 STEREOTROPIC REACTIONS 



base. The posterior part of the body and the tail are shark-like 

 in appearance. The eyes are freely movable and can be elevated 

 or retracted in a manner quite similar to the eye movements of the 

 frog. Specimens 3 to 4 feet in length may be taken, but the 

 reactions about to be described are better seen in the smaller animals, 

 15 to 18 inches long. 



When this animal is placed on a shark board and supplied with 

 plenty of aerated sea water through a rubber tube, little or no tying 

 is necessary to keep it in position. Under these conditions a contact 

 stimulus applied to the upper surface of the head or snout excites 

 certain very definite coordinated movements of the fins and eyes, 

 the particular combination of movements depending on the locus 

 and strength of the stimulus. 



II. 



If the skin of a Rhinohatus is gently stroked with the finger or with 

 a blunt instrument at any point along the midline of the head, for 

 example, between 7 and 8 (Fig. 1) , both eyes are retracted, the move- 

 ments of the two being approximately equal. If a similar stimulus 

 is applied near the outer margin of the upper surface of the head, 

 as at J (Fig. 1), the eye on that side is retracted strongly, the other 

 eye is moved very little or not at all. If trials are made at other 

 places, e.g., at 2 or J, Fig. 1, it is seen that as the point stimulated 

 approaches the midline the amount of movement of the two eyes 

 becomes more and more nearly equal, or in other words, the relative 

 amount of retraction of each eye varies inversely with its distance 

 from the point of application of the stimulus. 



It was relatively easy to record these movements graphically. 

 An Engelmann pincette was attached to each eye by a fold of 

 the integument just where the rudimentary lid passes over into the 

 cornea. The pincettes were connected by threads to a pair of light 

 heart levers in such a way that retraction of an eye gave an upward 

 direction to the curve. In the tracing here reproduced. Fig. 2, the 

 upper lever was connected with the left eye and the lower with the 

 right. The writing points were placed as nearly as possible in the 

 same vertical line but in order to make the relations more certain 

 simultaneous ordinates were marked throughout. The small rhythmi- 



