128 HISTOLOGY 



made up of larger cells, and fits, as the. real lens, on all the other layers 

 combined. The reticulum is composed of meshes that are drawn out 

 in the proximo-distal axis of the entire organ and in the direction of 

 light transmission. If the spaces in the reticulum are filled in life 

 with a fluid, especially if that fluid is of a high index of refraction, as 

 it probably is, the lens would act, as a whole, very efficiently. The 

 central axis of this organ is directed at a rather sharp angle to the body 



surface. It is possible 

 that, in life, this angle 

 can be changed at the 

 animal's convenience. 



One more cephalopod 

 light-organ must be de- 

 . .- scribed as an example 

 1 jyW of the highest complexity 

 and specialization. This 

 is the luminous organ 

 found in the integument 

 of Abraliopsis (Fig. 118). 

 This organ occupies a 

 FIG. 1 1 8. Section through principal axis of light-organ of nearly spherical sac near 



Abraliopsis. lu.c., luminous cells; I., lens; ref., reflector; the Surface, and has rep- 

 pg.c., pigment cells. (After C. CHUN.) , . . . 



resented in its composi- 

 tion all the structures that are found in almost any light-organ. Its 

 symmetrical central axis, passing from proximal to distal end, lies 

 at right angles to the body surface. A crescent-shaped blood space 

 occupies the posterior part of the sac. This space is divided into 

 compartments by a number of thin connective-tissue membranes. 

 The possibilities of blood circulation through these lacunae are not 

 known. 



Lying between this blood space and the other central organs is a 

 layer of pigment cells. They are symmetrically placed, one in the 

 central axis and the other two slightly overlapping it at the side. 

 Together they represent a cup-shaped figure, just covering and embrac- 

 ing, proximally, a mass of plasma which contains between its two parts 

 the strangely formed reflectors which, like the pigment elements, are 

 three in number. 



These reflectors are a puzzle in that their positions do not seem to 

 be mechanically adapted for the best or even for good results. They 

 are each composed of a plate of parallel, flat cells. In the middle one, 

 these cells are curved into a semicircle, which would be a shape of 

 reflector not well calculated to direct the rays all outward. The two 

 lateral bundles in our figure represent a circular reflector, which also is 



