GREEN PIC4MENT OF INTESTINAL WALL OF CIIiETOPTEllUS. 457 



mannas chart (PI. 36), not to a difference in the position of the 

 points of maximum absorption, but to a difference in tlie 

 position of the points of maximum luminosity, and consequently 

 of the area and graduation of the absorption around its maxima. 

 This is very difficult to represent or record by shaded drawings, 

 but is given with absolute precision by Professor Engelmann's 

 beautiful method of observation and record, of which I will 

 give some explanation below. The alkaline solution can be 

 rendered again neutral or acid, and the process reversed and 

 repeated indefinitely. 



III. Colour and Absorption Spectra of Bonellin. 



I had been anxious to compare the absorption spectra of Bo- 

 nellin and Chsetopterin for myself, and after I obtained a supply 

 of the latter was unable for some time to procure the former. 



I heard, however, in 1896 that Bonellia was flourishing in 

 the beautiful healthy tanks of the Laboratoire Arago,erected and 

 directed by Professor Henri de Lacaze Duthiers at Banyuls-sur- 

 Mer, near Perpignan. I accordingly wrote to that distinguished 

 zoologist, stating my desire to examine living specimens of 

 Bonellia in Oxford. With a kindness and courtesy for which 

 he is universally known and beloved. Professor de Lacaze 

 Duthiers sent to me from Banyuls, by express parcels-service, 

 two bottles of sea water, containing each a magnificent speci- 

 men of Bonellia viridis, which arrived in Oxford in a 

 perfect condition of living vigour. I was thus able to examine 

 again the pigment Bonellin, and to satisfy myself as to the 

 position in which it occurs in the body of Bonellia. My best 

 thanks are due to Professor de Lacaze Duthiers, and are here 

 recorded, for his great kindness. 



Greef has already correctly described the mode of occurrence 

 of the green pigment of Bonellia. It is distributed in the super- 

 ficial ectodermic epithelium in the form of very fine granules 

 which give the ectodermic cells a grass-green appearance. It 

 also occurs as fine granules in clusters of subepidermic cells 

 apparently belonging to the connective tissue. 



