190 EYES IN BIVALVE MOLLUSCA chap. 



the siphon, the majority are placed in grooves at the base of the 

 siphonal tentacles, forming an intensely black band round them. 

 A higher stage still is shown in Solen vagina, S. ensis, and Mactra 

 solidissima, where the cells are situated only in the siphonal 

 grooves, w^hich are more or less specialised in numbers and com- 

 plexity. 



Area Noae, according to Patten, is very sensitive to any 

 sudden change in the amount of light falling upon its mantle- 

 edge. A faint shadow cast upon it by the hand is suf&cient to 

 cause it to close its valves quickly, but always one or two 

 seconds afterwards, the promptitude in all cases depending upon 

 the depth of the shadow. Sensitiveness in this direction was 

 found to depend greatly upon the vitality of the animals them- 

 selves, since it always became less in those specimens which had 

 been kept for long in confinement. A shadow was not always 

 necessary to make them close. An ordinary black pencil, if 

 approached within two or three inches with extreme caution, 

 produced the same result, while a glass rod brought within the 

 same distance, and even moved rapidly to and fro, appeared to 

 cause no alarm. Sensitiveness to change in intensity of light 

 was experimentally noticed by the same author in the case of 

 Ostrea, Mactra, Avicula (to a special extent), and Cardium. It 

 is very remarkable to find that increased elaboration in the 

 structure of the eyes does not necessarily carry with it increased 

 sensitiveness, i.e. higher visual powers. Avicula, which is only 

 provided with a few scattered ommatidia, which would entirely 

 escape the notice of any one who had not seen them better de- 

 veloped elsewhere, was considerably more sensitive to light and 

 shade than Area, with its eyes of conspicuous size and much 

 more perfect organisation, instantly contracting the mantle 

 upon the impact of a shadow so faint as to be invisible to the 

 experimenter.^ 



Visual Faculties of Solen and Ostrea. — The visual power of 

 Solen may be exemplified by any one who is walking along 

 almost any of our sandy bays at extreme low-water mark. If 

 the day be warm and sunny, numbers of Solen will be seen 

 raising themselves an inch or two out of their holes ; but if you 

 wash to catch them you must approach very cautiously, and on 

 no account allow your shadow to fall upon them, or the}' will 

 1 W. Patten, Mitth. Zool. Stat. Neap. vi. (1886) pp. 546, 605 f. 



