282 GILBERT C. BOURNE. 



these and the large columnar pigmented cells at tlie margin 

 of tlie cup is well shown in 6g. 21. The preservation of my 

 specimens was not good enough to allow of my working out 

 such delicate details as the nerve supply of these organs. In 

 every case a nerve derived from the circumpallial nerve could 

 be detected in close proximity to an eye, and in fig. 20 two 

 such nerves are seen in section, one on each side of the optic 

 stalk. But I have been unable to trace nerve-fibres running 

 into those columnar pigmented cells which, because of their 

 characters and position I have called retinal. Further inves- 

 tigations are necessary before the exact nature and function 

 of these ''eyes" can be determined with certainty. But 

 their structure points to their being photoscopic, or possibly 

 thermoscopic. I have already alluded to their somewhat 

 paradoxical position and suggested that they may serve to 

 warn the animal to keep the valves of the shell closed during 

 the day-time. 



The Byssus and Byssus-gland. — In looking through 

 the literature of the subject one notes with surprise the 

 controversy about the nature of the byssus in Anomia 

 ephippium. The calcareous plate or ''ossicle" was a 

 stumbling-block to many of the older authors, who regarded 

 it as a third valve of the shell, and, though de Ijacaze 

 Duthiers gave cogent reasons for regarding the ossicle as a 

 calcified byssus, and Moore (12) described the fixation of the 

 young forms and the modification of the left valve produced 

 by the asymmetrical attachment of the byssus, neither of 

 these authors gave any account of the microscopical structure 

 of the byssus gland, and as recently as 1878 von Jhering 

 emphatically denied any homology between the "Falten- 

 organ " of Anomia and the byssus-cavity of other Lamelli- 

 branchia. 



The question was finally decided by Barrois (1), who 

 showed that the macroscopic and microscopic structure of 

 the " Faltenorgan " is in all essential particulars identical 

 with the byssus cavity of Area tetragona. But Barrois 

 contented himself with two very diagrammatic woodcuts of 



