Visual Organs of Insects and Crustacea, 363 



and the rectum {7ri), opening into a considerable cavity, which 

 has the Hver underneath, and the membrane enveloping the 

 spire above it. When the animal contracts the distended 

 foot, the water is seen to flow out between the mantle and the 

 shell on the right side. The tube and cavity are easily inflated 

 by a blowpipe introduced into one of the tubes of the foot. 



The moderately distended foot can scarcely be retracted 

 within the margin of the shell ; and, when fully injected, it is 

 elastic, and of a very large size. The cavity which it opens 

 into the sand is therefore fully adequate to receive the shell, 

 which is drawn down into it by the contraction of the muscle 

 of the spire. From the attachment of this muscle, the spu'e 

 is the part more directly acted upon, and which is depressed 

 in the greatest degree. Hence the notch is always uppermost; 

 and the ^uccinum, when completely buried, is enabled to 

 communicate with the water by its respiratory siphon.* The 

 habits of the Cassides, or helmet-shells, and of several others 

 amongst the carnivorous tribes of univalves, are similar to 

 those of the whelk, and they have probably a foot of similar 

 structure. 



I am. Sir, &c. 



G.J. 



Art. XIII. An Account of the Discoveries ofMiiller and others in 

 the Organs of Vision of Insects and the Crustacea. By George 

 Parsons, Esq. 



(Concluded from p. 234.) 



Vision of Insects and of the Crustacea, as produced 

 by the Compound Eyes, 



The visual circle of insects, &c., comprises only that part 

 of the whole horizon which corresponds to the extent of sur- 

 face of the eye; in other words, insects see those objects 

 alone which are situated perpendicularly to some part within 

 the circumference of the cornea : and, as their eyes undergo 

 no changes of form, they consequently have no power of 

 adapting themselves to the vision of objects placed at differ- 

 ent distances ; they see clearly and distinctly only the nearest 

 objects. 



The compound eyes are neither dioptric nor catoptric 

 instruments : animals provided with them see only by means 



thrown below and to the right side of its natural situation, to allow the 

 . opening of the tube to be seen j e, the respiratory trunk ; Ar, the origins of 

 the muscles of the boring trunk ; /, the course of the tube by which the 

 foot is supplied with water; ?«, its termination." (Phil. Trans, for 1826, 

 pl.xiv. fig.3.) 



