248 A. M. Banta 



Since Caecidotea stygia and Asellus communis are so closely 

 related, they are appropriate species for comparison. Packard in 

 his comparison of the two species showed them to be much alike 

 structurally, indeed, he ('88, pp. 29-33) supposed Caecidotea 

 stygia to have been derived from Asellus, but placed it in a dis- 

 tinct genus because of its laclc of eyes and its more slender body and 

 appendages. Miss Richardson ('05, p. 4io)hasmadeuseof only the 

 characters Packard had proposed as a basis for separating the two 

 genera. Asellus occurs in the cave regions of Indiana and, at 

 present at least, has the same opportunity to be a cave inhabitant 

 that Caecidotea has. Asellus communis, unlike Caecidotea, is 

 pigmented about as fully as most crustaceans. Its eyes consist 

 of from 12 to 20 irregular facets compacted together. 



There is not an obviously greater development of the tactile 

 organs about the head of Caecidotea than about that of Asellus. 

 The much more slender and flattened body and the longer and 

 more slender antennae and legs of Caecidotea, however, would 

 apparently contribute to greater sensitiveness on its part. 



I. HORIZONTAL ILLUMINATION 



I. Methods and Apparatus 



The experiments with light were carried on in the basement of 

 the Museum of Comparative Zoology in a west room, which could 

 be made dark or arranged to admit either difi^'use daylight or direct 

 sunlight as desired. 



Most of the experiments were made with artificial light, during 

 which of course daylight, as well as direct sunlight, was excluded 

 from the room. A glass tank (compare Fig. i,p.250, for the arrange- 

 ment of the whole apparatus) 51 cm. long, 22.6 cm. broad and 7.7 

 cm. deep, inside measurements, was used to confine the animals 

 during experimentation. Its sides were of plate glass 5.8 mm. 

 thick and its bottom was a removable sheet of glass 3 mm. thick 

 with a ground upper surface. For convenience in making records 

 of the experiments, the side walls of the tank were divided into six 

 equal sections indicated by vertical lines. These sections are 



