PHOTOSPHERIA OF NYCTIPHANES NORVEGICA. 331 



stage the actual development of the organs, and so test the 

 above hypothetical account, but we have examined the con- 

 dition of the posterior organs in very young specimens, which 

 were only a quarter of an inch in length, but which were 

 already similar in external characters to the adult. In these 

 young specimens the organs already possess all the different 

 parts present in the adult organs, but in a somewhat embryonic 

 condition. A section of one of these organs is shown in fig. 5. 

 It is from one of the anterior thoracic pair. It will be seen 

 that the continuity of the cornea and of the posterior cell 

 layer with the epidermis is well shown, and the whole organ is 

 evidently arising by differentiation in an epidermic thickening. 

 The fibrillar mass is different in appearance from that in the 

 adult, the striations being all parallel to the principal axis of 

 the organ. The cells of the posterior cell layer still retain 

 their primitive character. Fig. 6 shows a similar section of the 

 first abdominal organ at the same stage. 



Function of the Organs. 



Nearly everybody who has written about the luminosity of 

 the Euphausiidae has mentioned that the emission of light is 

 intermittent, and is directly affected by stimulation. It is 

 usually also stated that the activity of the photospheria is under 

 the control of the will of the animal, and is soon exhausted by 

 repeated exercise. Our observations go to confirm these state- 

 ments, but we regret to say that they are not altogether con- 

 clusive, and it is desirable that experiments more rigidly exact 

 should be made on the living animals. The observations are 

 as follow: 



The behaviour of the animals when alive, and, as far as can 

 be judged after capture, in a healthy normal condition, is pecu- 

 liar. They are in a state of almost incessant activity, swim- 

 ming restlessly forwards and struggling vigorously against any 

 obstacle they come into collision with. Their motion is due 

 almost entirely to the limbs, the sudden backward movement 

 produced by flexion of the abdomen is scarcely ever observed. 

 And it is curious to note that they are as often on their dorsal 



