104 w - J- CROZIER. 



"steps" of this sort, perhaps three in all as a rule, result in an al- 

 most perfect alignment of the median plane in the axis of the light 

 beam; the Lima then swims, Pecten-wise, in a straight line away 

 from the light. The swimming act is not usually prolonged, 

 perhaps 50 cms. representing the maximum distance covered 

 before temporary exhaustion intervenes, but the orientation 

 process is of a definite, clean-cut character and was previously 

 unknown, so far as I am aware, among bivalves. It is a point 

 of some interest that in a photosensitive form such as Lima, 

 where structural conditions give the possibility of phototropic 

 behavior, we do in fact find such behavior diagrammatically 

 exhibited. 



The process of body-orientation is usually as just described; 

 but if light be directed upon the gape of a vigorous Lima it some- 

 times gives a violent somersault, turning completely through 

 1 80 by some mechanism which I was not able to analyze; the 

 subsequent swimming is in the usual manner. 



I tried to find out if the foot, so active in the commoner 

 orientation process, were itself photosensitive. After various 

 trials with local illumination I came to the belief that it was, 

 but complete proof of this I did not secure. 



It is especially when the animal is weakened by handling or 

 by laboratory conditions that swimming on the side is found. 

 When in good condition a Lima can swim for at least 25 cms. 

 through the water without touching substratum. 



The photonegativity of Lima, involving so curious a method of 

 orientation, is obviously important in maintaining these animals 

 in crannies and darkened clefts, under stones and among coral 

 heads. 



Summary. The bivalve Lima is photonegative, and in vigor- 

 ous individuals the response to illumination involves a true and 

 accurate orientation of the body, with gape away from the light, 

 by means of the long adhesive foot, which is itself perhaps photo- 

 sensitive; orientation is followed by Pecten-like swimming, the 

 valves being vertical. 



CITATIONS. 

 Bauer, V. 



'12 Zur Kenntnis der Lebensweise von Pecten jacob&us L. Zool. Jahrb., Abt. 

 allg. Zool. u. Physiol., Bd. 33, pp. 127-150. Taf. 6. 



