164 EYES OF MOLLUSCS 



In order to isolate pieces of the cornea with the subjacent 

 pseudo-cornea and the circular fibres on the outer surface of the 

 lens, it is better to macerate the eyes in sulphuric acid as given 

 above. The same treatment retains to perfection the natural 

 shape of the lens, which may then be isolated and its surface 

 studied to advantage. 



It is necessary for the study of the circular retinal membrane^ 

 the septum, and the retina itself, to isolate the latter intact. 

 Maceration in chromic acid either makes the retina too brittle or 

 too soft, while the axial nerve-fibres remain so firmly attached to 

 the retina that it is difificult to isolate it without injury. But this 

 may be easily and successfully done by maceration for one or two 

 days in the sulphuric acid solution. By this treatment the retina, 

 together with the septum and circular retinal membrane, may be 

 detached entire. 



Surface views of the retina show the peripheral outer ganglionic 

 cells. The argentca may be very easily separated in large sheets 

 by macerating for four or five days in bichromate of potash of one 

 per cent. 



Sulphuric acid is a most valuable macerating as well as 



preservative reagent. In weak solutions (forty drops to fifty 



grammes) entire molluscs, without the shell, have been kept in a 



perfect state of preservation for more than six months. For cilia 



and nerve-endings it is exceptionally good. 



The eyes of Area and Pectunculus may be macerated either 

 in Miiller's fluid or chromic acid. Undiluted Mliller's fluid in 

 twenty- four hours gives more satisfactory preparations than a weak 

 solution which is allowed to act for a longer period. Chromic 

 acid, one-fifth per cent, for ten or twelve days, gave most of the 

 preparations from which the drawings of the nerve-endings were 

 made. A few drops of acetic and osmic acid added to distilled 

 water give a very energetic macerating fluid for the epithelium of 

 marine molluscs. Such preparations led to the discovery of the 

 very delicate outward continuations of the pigmented cover-cells 

 in the compound eyes of Area. 



II. — Arthropods. — In order to demonstrate the presence of 

 the corneal hypodermis in the faceted Arthropod eye, and the 

 connection of the so-called " rhabdom " with the crystalUne cone 



