454 SUPPLEMENTARY NOTES. 



the approach of danger, indicates the possession of a sense ana- 

 logous, at least, to that of ordinary vision. These eye-specks may 

 be seen in the Pecten, placed at short intervals round the thick- 

 ened edge of the mantle, on the outworks, as it were, of the 

 internal part of the animal fabric. ' As locomotion so vision ' is a 

 general aphorism not without its particular exception; for there 

 is good reason for believing that Spondi/lus, which is a fixture in 

 its adult state, is furnished with these visual specks." (Penny 

 Cyclopaedia, vol. vii. p. 432, et seq. Article Conchifera.) Ehren- 

 berg has described the eyes of the Medusa aurita to be in the 

 form of minute red points on the circumference of the disk. He 

 has also ascertained the existence of small red eye-specks at the 

 extremity of the rays of the Asterias. 



P. 248. The specific gravity of a body, is its weight, com- 

 pared with the weight of another body, whose magnitude is the 

 same; hence, if a body which occupies any given space in water 

 be contracted into a smaller magnitude, whilst its absolute weight 

 remains the same, it becomes specifically heavier. Supposing 

 the absolute weiglit of the body of the Nautilus, and also that of 

 its pericardial fluid, to be the same as that of an equal bulk of 

 water, the body, when immersed, would always displace a 

 quantity of water, equal to its bulk. The presence of the peri- 

 cardial fluid within the body, (i. e. within the Pericardium,) or 

 its removal from it into the shell, would not aff'ect the specific 

 gravity of the body, because the magnitude of the body varies 

 according as the pericardium is either empty, or distended with 

 its peculiar fluid. But, as the magnitude of the shell is con- 

 stantly the same, whilst the quantity of matter within it varies, 

 as the pericardial fluid enters or leaves the siphuncle, its specific 

 gravity is varied accordingly, being increased, when the fluid 

 enters the siphuncle (compressing the air within the air-cham- 

 bers,) and diminished, when this fluid returns from the siphuncle 

 into the body. 



When the animal, preparing to rise, emerges, from its shell, and 

 the pericardial fluid, returning from the siphuncle into the peri- 

 cardial sac, enlarges the body by the distension of this sac, the 

 absolute weight of the body and shell together remains the same, 



