FOOD OF SALP^E. 167 



ball, which, being opened, is found to be filled with 

 Foraminifera and Diatomacece, from which latter it 

 derives its colour. A very large species of Salpa, 

 measuring some six or seven inches in length, is found 

 in the equatorial regions of the Atlantic, whose pro- 

 portionally larger digestive cavities are filled with 

 Hhizoselenia, a tubular form of Diatom occurring in 

 vast profusion there. " The alimentary matter of the 

 Salpce" observes Mr Macdonald, " is composed of ani- 

 mal and vegetable* elements in nearly equal propor- 

 tions ; and when the microscope reveals the calcareous 

 shells of Foraminifera, the beautifully sculptured 

 frustules of Diatomacece, keen siliceous needles [of 

 Sponges], and the sharp armature of minute Crustacea, 

 within an intestinal tube so tender and friable that it 

 withers at the human touch, one cannot help admir- 

 ing the operation of those conservative properties with 

 which its delicate tissues are endowed. Each atom 

 yields to acute impression as by an instinctive intelli- 

 gence, evading injurious contact ; and although a con- 

 tractility of the tube is essential to the due perform- 

 ance of its functions, no evil thus befals its integrity 

 tin the term of life is at an end." 



The digestive action of the Mollusk effects no change 

 in the earthy constituents of its food ; and thus the 

 calcareous shells, and the siliceous spicula, and frus- 

 tules, lie uninjured in its stomach, disjointed and 

 broken, perhaps, by trituration, and cleaned of all 



* That is, assuming the Diatomacece to be plants, according to 

 the received doctrine ; but vide supra. 



