146 



CRUSTACEA EUCARIDA 



as in the type figured, upon the outer margins of the stalked 

 eyes, on the bases of the second and seventh thoracic limbs, and 

 on the ventral median line on the first four abdominal segments. 

 These organs are lantern-like structures provided with a lens, a 

 reflector, and a light-producing tissue, and they are under the 

 control of the nervous system. Their exact use is not known, 

 any more than is the use of phosphorescence in the majority of 

 organisms which . produce it ; but in certain cases it appears that 

 the Euphausiids make use of their phosphorescent organs as 

 bull's eye lanterns for illuminating the dark regions into 

 which they penetrate or in which some of them permanently 



Fia. 102. — Euphausia pellucida, female, x 5. (?, Last gill ; L, liiminou.s organ of first 

 leg ; L'y Inminous organ of 2nd abdominal segment ; T, biramous thoracic 

 appendages. {After Sars. ) 



dwell. At any rate, associated with the presence of these organs 

 in some deep-sea Euphausiids are remarkable modifications of 

 the eyes ; and we may perhaps here fittingly introduce a short 

 discussion of these visual modifications in deep-sea Crustacea, 

 and the conditions which call them forth. 



The compound eyes of Crustacea resemble those of Insects 

 in that they are composed of a very large number of similar 

 elements or " ommatidia," more or less isolated from one another 

 by pigment. Each ommatidium consists typically of a corneal lens 

 (Fig. 103, c), secreted by flat corneagen cells (c.ff) below ; beneath 

 the corneal lens is a transparent refractive body called the " crystal- 

 line cone " (cr), which is produced by a number of cells surround- 

 ing it called the " vitrellae " (v). Below the crystalline cone 

 comes the " rhabdom " (rh), produced and nourished by " retinula- 



