COLE. — IMAGE-FORMING POWERS OF VARIOUS TYPES OF EYES. 361 



results, by exposing the deep-lying cells — which are supposed to be 

 the organs stimulated — unequally to the light. Undoubtedly, too, 

 these animals are not the simple mechanisms that we are often inclined 

 to consider them, but are subject to certain internal conditions — 

 physiological states, if we care to call them so — which are beyond 

 our observation and control, and by their changes produce corre- 

 sponding irregularities in the results. On this account it is important 

 to use animals for experimentation that have previously been sub- 

 jected for a considerable period to constant conditions. 



2. Land Planarian {BipaVmm keicense Moseley). 



The so-called eyes of planarians are remarkably uniform throughout 

 the group in general plan of construction. Each consists of one to 

 several sense elements, or cells, backed by a pigmented cup, which is 

 in turn composed of one or a few cells. These eyes have no appa- 

 ratus for the formation of an image, but are constructed in such a way 

 that generally light fi-om only one direction can affect a single eye at 

 a given time. For this reason they are often spoken of as directum 

 eyes. In the genera Planaria, Dendrocoelum, and related forms, only 

 two eyes are present; but in others, such as Polycelis and the laud 

 planarians, eyes may be present in great numbers. 



Bipaliinn is a well-known genus, comprising a large number of spe- 

 cies, mostly indigenous to Japan, China, India, Ceylon, and the Malay 

 Archipelago. There is well established in certain greenhouses in Cam- 

 bridge a species, the original home of which is unknown. This species 

 was first described by Moseley (78) from a specimen obtained in the 

 Kew Gardens, England, and was named by him BipaUiim keivense. 

 Since that time it has been found in hothouses in widely separated 

 localities in nearly all parts of the globe, and it is generally believed 

 to have been unintentionally distributed with consignments of plants, 

 either from its original home or from other places where it had become 

 established. Woodworth ('96) reported it from Cambridge, Mass., in 

 1896. A form described from Landsdowne, Pa., by Sharp ('91) as 

 Bipalium manubriatum is probably the same species ; and Wood- 

 worth (^98) has since reported it from other localities in the United 

 States (Baltimore, Md., Pittsburg and Allegheny, Pa., and Spring- 

 field, Ohio). 



Bipalium keivense is the largest of the land planarians, the speci- 

 mens ordinarily ranging from 12 cm. or 13 cm. to 25 cm. in length; 

 and Woodworth ('96) records one 30 cm. long. These measurements 

 are those of the worm in the extended condition, which is assumed in 



