2l6 'Journal of Comparative Neurology arid Psychology. 



With the foregoing facts in mind, it is clear that the problem before 

 us becomes, not, what reactions do the larval lobsters in general 

 give to light, but how do the lobster larvce of a certain age react to 

 light under certain known conditions? To this rather more com- 

 plex question attention will now be given. 



V. SYSTEMATIC ACCOUNT OF THE REACTIONS TO LIGHT OF 

 LOBSTERS IN THE LARVAL STAGES. 



What is the nature of the reactions to light through the succes- 

 sive developmental stages, and by what conditions is it determined .^ 

 Regarding the first of these points, it should be borne in mind 

 that the subject matter concerned cannot be treated concretely, 

 but that it is necessarily scattered through the long series of observ- 

 ations which follows, and that it is only from a consideration of the 

 series as a whole that a clear idea of the gradual modifications in 

 the reactions from the first to the sixth stage of the lobster's life 

 can be obtained. As to the second point of inquiry, it is at once 

 perceived that the conditions or factors which we seek to discover 

 are of two sorts : 



1. Conditions which are peculiar to a certain definite age or 

 stage in the development of the larva, and which may be designated 

 as physiological conditions. 



2. All outside influences, including the intensity and multi- 

 plicity of stimuli brought to bear upon the animals. 



In the following discussion it will be found of advantage to con- 

 sider these two kinds of modifying conditions together; for they are 

 found to be very much inter-related when a consideration of their 

 mutual importance in bringing about any orientation of the young 

 lobsters is involved. 



It may be appropriate to mention at this point the method of 

 securing the data here presented. The futility of taking young 

 larvae at random from the hatching bags without knowledge of their 

 age or previous history was recognized early in the course of the 

 investigation. It was considered advisable to work only with those 

 lobsters whose previous history was definitely known. To this 

 end the exact time of hatching of certain groups of larvae was noted. 

 In the large canvas hatching bags, used at the Wickford Station, 

 hundreds of larvae hatch in a single hour, and observations were 

 made, as a rule, twice each day (morning and afternoon), upon 



