266 -Prof. P. M. Duncan on the 



known to have a proper nervous system ; their optic nerves, where they 

 exist, are apparently isolated, and not connected with a nervous ring 

 such as exists in the higher Radiate animals." 



II. A Description of t7ie Morphology of the Chromatophores. 



During the summer of 1871 the author of this communication was 

 examining into the minute anatomy of Actinia mesembryanthemum, and 

 had the advantage of possessing living specimens. Having satisfied 

 himself of the general correctness of Botteken's admirable work, he 

 relinquished the inquiry until 1873, when he resumed it. 



Every one who has endeavoured to anatomize one of the Actiniae must 

 acknowledge the excessive difficulties which accompany the attempt. 

 The irritability of the muscular tissues, their persistent contraction during 

 manipulation, the confusion caused by the abundance of different cellular 

 histological elements, and the general sliminess of the whole, render the 

 minute examination very troublesome and usually very unsatisfactory. 

 Reagents are useful for rough examinations ; but when the most delicate 

 of the tissues are to be examined they must be floated under sea-water, 

 and this must be the medium in which they must be examined under the 

 microscope. Carmine-solution, osmic acid, and spirits of wine in weak 

 solutions are useful after the natural appearances have been determined, 

 but they exaggerate some histological elements and destroy others. 



Great care must be taken in making the thin sections, and no tearing 

 must be allowed ; for it is of paramount importance, in endeavouring to 

 trace the nervous system, that the relative position of parts should be 

 retained. 



It is useless to rely on any observations made with object-glasses 

 lower than jJg-inch focus (immersive). 



In examining the chromatophores, Actiniae with very bright-coloured 

 ones, and other specimens with these organs dull in tint, should be 

 selected. Fresh subjects should be obtained, and it is not necessary to 

 kill them first of all. The blades of very delicate scissors should be 

 allowed to touch the desired chromatophore close to its base, and then as 

 the Actinia commences to contract, they should be brought together 

 gently and without wrenching the tissues. By this method the chroma- 

 tophore will remain on the blades. Two or three chromatophores may 

 be removed, with their intermediate tissues, without injury to the animal ; 

 but, of course, the excision must not be too deep, or the endothelium will 

 be cut into. 



A dropping-tube should be used to wash the chromatophore off the 

 blades on to a glass slide, where a drop of sea-water awaits it. 



Sections are by no means easy to make, but they are best performed 

 under a power of 10 diameters with fine scalpels. The forceps must not be 

 employed, as it crushes the tissues. If possible, very slight pressure 



