RANSACTIONS OF SECTION D. — DEPT. ANATOMY AND PHYSIOLOGY. 419 



more or fewer, sometimes even more than half of the arms, which come off from 

 the aboral side of the disc, have no ambulacral nerve at all, although the dorsal 

 axial cord gives off its two pairs of branches in the usual way. In one large 

 species from the Philippines, with nearly 200 arms, this condition is not limited to 

 the aboral arms only, but occurs on some of the arms on each radius, while the 

 others have the usual groove and subjacent ambulacral nerve. 



These facts are strongly indicative of the nervous nature of the axial cords, 

 although Olaus and Gegenbaur in their recently published text-books make no 

 mention of this view at all, and describe the nervous system of Comatula as essen- 

 tially similar to that of the starfishes. It would seem, however, that while the 

 ambulacral nerve of the Ophiurids supplies the muscles as well as the tentacles, 

 these functions are more differentiated in the far more active Crinoids. The axial 

 cords of this group appear to be the principal motor nerves as far as the skeleton 

 is concerned, while the ambulacral nerves supply the tentacles only, possibly having 

 some influence on the slow creeping movements which the isolated disc has been 

 observed to perform. Why should we deny the nervous nature of the axial 

 cords, simply because our doing so would clash with our preconceived notions as 

 to what the Crinoids ought to be, in order to agree with the views on Echinoderm 

 morphology which were adopted without a sufficient knowledge of the anatomy 

 of this most interesting group ? 



5. On a Visual Phenomenon and its Explanation. 

 By William Ackkotd, F.LG. 



Visual phenomena are of general interest, and have been noticed by Brewster, 

 Herschel, and others. The following is the visual phenomenon in question: a 

 globule of water is made to impinge on the cornea, whilst the gaze is unwinkingly 

 fixed on a distaut light. Directly after the impact the light appears to be sur- 

 rounded by a luminous ring, which gradually contracts in diameter. Explana- 

 tion: A minute ripple is produced on the surface of the cornea. The crested 

 wave-ring and the refracting media of the eye produce two hollow cones of light 

 within the vitreous humour, one with a circular portion of the crystalline lens as 

 base, and the other with the retina as base. As the ripple increases in diameter, 

 the first cone increases in size, and its prolongation (the second cone) diminishes, 

 and its base (in other words, the visible luminous ring), becomes less and less, 

 until it merges into the lamp-light itself. 



TUESDAY, AUGUST 26, 



The Department did not meet. 



e e 2 



