736 SUMMARY OF CURRENT RESEARCHES RELATING TO 



edule which appear to be correlated with the conditions of life. He 

 regards as the most important result the fact that the shells of each 

 sample, whether it be from a separate lake or only from a particular 

 level, have special characters, and are more like to each other than to 

 the shells of one of tlie other lakes or of another level. Again, the 

 shells which have lived under similar conditions, i. e. in very salt water, 

 resemble each other, having the characters of thinness, light colours, 

 small beaks, ribbing on the inside of the shell, and great relative length. 

 Similarly, the shells from the two isolated and independent fresh-water 

 lakes at Eamleh also present similar characters, viz. thickness, similar 

 texture, and shape. It is to be noted that the resemblance between the 

 cockle-shells from an Asiatic lagoon and those from Abu Kir becomes 

 still more striking when it is remembered that their immediate ancestry 

 is very different ; the Asiatic shells had been living for many generations 

 in the brackish water of the Aral Sea, and had already become a well- 

 marked variety before being subjected to the new conditions, while those 

 which are found in Abu Kir must clearly be the immediate descendants 

 of animals of the type found in the Mediterranean. The author suggests 

 that in so far as any variation (as, for example, that of texture) occurs 

 universally among the shells of a given sample, it may be legitimately 

 supposed that they are correlated to the conditions under which they 

 lived. The terraces of Shumish Kul give an opportunity for the com- 

 parison of several distinct stages in the origin of a natural variation, 

 which appears to be almost unique. 



Luminous Phenomena in Pholas dactylus.* — M. E. Dubois . has 

 been much struck with the sensitiveness to light exhibited by the eyeless 

 Pholas dactylus. The contractile warning apparatus is made up of 

 muscular segments which are merely the continuation of the pigmented 

 epithelial elements which form a continuous layer on the siphon, beneath 

 the cuticle. The pigmented segment and the muscular segment together 

 form the photomuscular element. This warning apparatus is in more 

 or less direct relation to the sensory elements of the periphery. When 

 a ray of light falls on the surface of the siphon (" photodermatic 

 retina "), it traverses the cuticle and exercises its action on the proto- 

 plasm of the pigmented segments. The modifications caused by this 

 luminous radiation also determine a contraction of the muscular segment. 

 This contraction disturbs the peripheral nervous elements just as if the 

 siphon had been excited mechanically by touching its surface, and pro- 

 vokes a reflex contraction analogous to that of the iris when a ray of 

 light strikes the retina. The mechanism of vision is, therefore, reduced 

 to a true tactile phenomenon. 



The constituent elements of the organs of Panceri, in place of being 

 covered by a refractive cuticle, carry vibratile cilia. They are formed 

 of a calyciform epithelial segment which is directly continuous with a 

 muscular segment, and make up the myophotogenic segment. In the 

 fresh state the calyciform epithelial elements are filled with a substance 

 which they extrude when excited ; in the midst of these, in the mucus 

 which has become phosphorescent, there are numerous migratory blood- 

 cells and the Bacterium pholas which the author has already described. 



The most striking point is the great resemblance in structure and 

 function between the parts which perform the photodermatic function 



* Comptes Eendus, cix. (1889) pp. 233-5. 



