362 PEOCEEDINGS OF THE AMERICAN ACADEMY. 



crawling. The body is capable of great contraction, so that a worm 

 which is, when crawling, 25 cm. long, may contract to a length of only 5 

 or 6 cm. (see figures given by Bell, '86, pi. 18). The head when ex- 

 tended is semilunar, or shaped like a battle-axe (see Figure U, p. 364), 

 and is about twice as broad as the body immediately behind it. The 

 eyes have been described in other species of Bipalium by Moseley 

 ('74, p 144) and in B. kewense by Bergendal ('87) and Janichen ('96, 

 p. 275; Taf 10, Figur 18; Taf 11, Figur 21). Bergendal ('87, p. 4it) 

 describes the position and arrangement of the eyes as follows : " Eyes 

 occur in this species in enormous numbers. They form a zone of three 

 or four rows near the margin of the head, and are also placed on the 

 sides (not on the back) of the whole body, even to the hindmost end." 

 The largest eyes are on the head and immediately behind it, and they 

 are closer together in those regions than on the body farther back. 

 According to Janichen ('96, p. 27G), each pigment cup has the form of 

 a hollow hemisphere, the convex side of which is turned inward, while 

 the flat side, which is open, is directed outward. The pigment spot, 

 which forms the background of the sensory cells, is composed of a sin- 

 gle cell. From three to six flattened sense cells lie in the pigment cup, 

 and, as in Polycelis, the eyes farther posteriorly are not only smaller, 

 but usually have a smaller number of sense cells. 



Hesse ('97) has discussed from a theoretical standpoint the manner 

 in which the eyes of planarians probably function. He deals in this 

 case with those forms which possess only two eyes. The diagrams 

 which he gives ('97, p. 575) are reproduced in Figure 7, and require 

 only a brief explanation. When there is only a single sense cell in the 

 pigment cup, it may be stimulated by light coming from various 

 angles, covering a considerable range, that is, by any light which is 

 able to enter the cup. Thus the effect would be the same in the left 

 of the two eyes represented in the diagram for light coming in any of 

 the directions shown by the arrows in A, B, C, or D. If, however, 

 there were a number of sense cells in the cup, only a part of these 

 would be reached by the light coming from any one of the directions 

 shown, and the light would strike different cells according to the direc- 

 tion from which it came. In the first case, the structure of the eye 

 would not furnish any opportunity for determining the direction of the 

 light within the range of the arrows in A to D ; in the second case it 

 would, since under the conditions shown in A the cells on one side of 

 the cup (posterior) would receive the light, whereas in J) those of the 

 other side would receive it. This condition may be considered as the 

 beginning of a crude image-forming apparatus, the eye approximating 

 in a rough way the concave mosaic type. These are theoretical deduc- 



