56 W. J. DAKIN. 



ordinary methods were more complete than by the complicated 

 ones, and there was usually a far greater freedom from 

 artefacts. There remains finally the maceration methods to 

 be referred to. The lens-cells, with all their peculiar 

 pi'ocesses, were easily isolated after immersion of the eyes 

 directly in a 3 per cent, solution of chloral hydrate in sea- 

 water for about four hours. The same solution was used for 

 the retinal cells, and the eyes were placed, as above, directly 

 into this medium. After two hours the retina was dissected 

 out from the eye, placed in a drop of water on a slide, and a 

 cover-glass supported by wax feet placed above it. Gentle 

 tapping on the cover-glass separated the elements. Chromic 

 acid solutions in sea-water of -g^^- per cent, strength gave very 

 good results for macerations of the rod-cells and rods. 



This was also used as advised by Patten after fixation of 

 the eyes in |- per cent, chromic acid for five minutes. 



The maceration preparations were examined unstained, and 

 stained with picro-carmine. 



The chief species examined have been Pecten maximus 

 and P. jacobseus, with the following others: Pecten oper- 

 cularis, P. varius, P. tigrinus, and P. tenuicostatus. 



Position and Number of Eyes. 



The eyes of Pecten occur on the mantle-edges of both 

 valves. The mantle-edge can be said to be divisible into 

 three folds, the periostracal fold, the ophthalmic fold, and 

 the velum (PI. 7, fig. 2, F.). All three possess tentacles, 

 those situated on the first two being long and mobile sensory 

 structures, well provided with sense-cells for the perception 

 of tactile and olfactory stimuli, whilst those on the velum 

 are short and rather immobile. 



The eyes are situated on the median fold, between the 

 periostracal groove and the base of the velum (PI. 7, fig. 2, 

 Eye), and amongst the long tentacles. Poli in 1795 noticed 

 a certain resemblance of the eye-stalks to the tentacles, and 

 considered them as modifications of the latter. 



The number of eyes present varies considerably for the 



