PHYSIOLOGICAL LIGHT. 421 



IV. 



Among the Mollusca, we find that cephalopods show luminosity only 

 in certain rare species, though there may yet be some doubt concerning 

 the function of certain organs supposed to be photogenic before their 

 action was observed in living specimens. Luminosity has been noted 

 in several molluscous gasteropods: Aeolis, Hyalea, Creseis, Cleodora, 

 Phyllirrho?, and in one lamellibranchiate, Pholas dactylus; it has been 

 carefully studied only in the latter and in the Phyllirrhce bucephala. 



Phyllirrhce is a naked, pisciform, Mediterranean gasteropod, having 

 a laterally compressed, transparent body. The light is produced in 

 peripheral, nervous cells of an ordinary form, in the cells of the central 

 ganglia, and in tegumentary elements having very marked, dark con- 

 tours and inclosing numerous rounded granulations immersed in the 

 intracellular liquid. 



The photogenic function again occurs in nervous and tegumentary 

 cells having granular contents in Pholas dactylus, a sedentary mollusk 

 that on our coast inhabits holes hollowed in the rocks, where it lives 

 secluded, showing only the end of its siphon — a kind of double-barreled 

 contractile tube by which it draws in and rejects the water that serves 

 for its nutrition. The external integument of the siphon is sensitive 

 to light like the retina of the human eye, with which it presents many 

 analogies; it is the seat of the dermatoptic function, 1 while the internal 

 wall of one of the two tubes of the siphon is the seat of the photogenic 

 function. There is also as much analogy between the structure of these 

 two walls as has been shown to exist between certain photogenic and 

 visual organs of Crustacea or fishes. 



In Pholas (fig. 9), the light originates in the nervous, internal subepi- 

 dermic coat of the aspirator siphon, and very likely, as in Phyllirrhce, in 

 the nervous elements that form a sort of diffuse ganglion; but, besides, 

 also like that gasteropod, it appears in the tegumentary elements with 

 granular contents, arranged, in Pholas, in the form of two glandular 

 cords and two triangles situated on the inner surface of the aspirator 

 canal. In response to various stimuli, there is formed in this canal 

 an abundant secretion of a highly luminous mucus that communicates 

 to the water and to bodies that imbibe it a beautiful bluish and quite 

 persistent luminosity. Microscopical examination shows in this mucus 

 various elements from the internal wall and from the blood, and espe- 

 cially certain cells with well-marked contours inclosing a liquid that 

 holds in suspension rounded protoplasmic granulations. Other granu- 

 lations of a similar nature, coming from the glandular caliciform cells 

 of the cords and from the triangles, swim abundantly in the luminous 

 mucus. We will study their metamorphoses in the section of this 

 article devoted to the special mechanism of the photogenic function. 



^See Nouvelle thdorie de mdcanisnie ties sensations luiniueuses : Revue general e ties 

 sciences pares et applique"es, avril, 1890. 



