PARKER. — THE SENSORY REACTIONS OF AMPHIOXUS. 423 



less transparent animal first penetrates the skin, and it is not impossi- 

 ble that the receptive organs for light really lie in this layer, as main- 

 tained by Nagel ('94^ p. 811) and Jelgersma (:06, p. 390). This 

 opinion is strengthened by what has recently been made out concern- 

 ing the sensitiveness to light of the skin of certain reptiles, amphibians, 

 and fishes, particularly ammocoetes (Parker, : 03*^, : 05''). 



Since I was unable to devise an experiment whereby the nerve-tube 

 in amphioxus could be illuminated without having the light pass through 

 the skin, I cannot be absolutely sure where the light-receiving organs 

 lie, but there is a certain amount of indirect evidence on this question, 

 all of which points in one direction. As has already been shown, the 

 skin on the anterior end of the animal is not sensitive to light, this 

 form of sensitiveness beginning posteriorly at no special region so far as 

 the skin is concerned, but exactly where the eye-cups first occur in the 

 nerve-tube. This evidence, so far as it goes, favors Hesse's view that 

 these eye-cups are the true light-receptive organs. Another piece of 

 evidence has to do with the exact distribution of the animal's photo- 

 receptiveness and that of the eye-cups. If different regions on the 

 length of a lancelet are tested for their sensitiveness to light, they will 

 be found to vary considerably. The most sensitive region is that 

 which extends from a point several segments behind the anterior tip of 

 the nerve- tube posteriorly over about one quarter of the length of the 

 animal ; the region next in sensitiveness is the most posterior quarter 

 of the animal ; and the least sensitive part of the whole region which 

 is at all sensitive is approximately the middle half. In a series of 

 trials in which was determined the relative intensity of the minimum 

 amount of light necessary to stimulate in these three regions, it ap- 

 peared that, if the minimum intensity for the anterior portion, the 

 most sensitive part, is called 1, that for the posterior part was 1.5, and 

 for the middle part 25.0, while an intensity of 0.5 was not stimulating 

 to any part of the animal. If, now, the distribution of the eye-cups 

 described by Hesse be taken into account, a striking correspondence to 

 the sensitiveness to light will be found. In Branchiostoma caribbaeum 

 the most anterior eye-cups occur in the third segment, and the remain- 

 ing cups form a more or less segmentally arranged series reaching to 

 the last segment of the body, which is practically the tip of the tail. 

 In this series, so far as numbers are concerned, three general regions 

 can be distinguished. The first region, the one in which the cups are 

 most numerous, extends from about the fourth segment to about the 

 twentieth ; the region second in abundance covers about the last twelve 

 segments of the body ; and the third region, or the one in which they 

 are fewest, is the middle portion of the body between the two regions 



