190 EYES IN BIVALVE MOLLUSCA chap. 



about over the surface of 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, which are more or less 

 specialised in numbers and complexity. 



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 sufficient 

 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 

 developed elsewhere, was considerably more sensitive to light 

 and shade than Arca^ 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 incli or two out of their holes ; but if you 

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

 no account allow your shadow to fall upon them, or they will 

 1 W. Patten, Mitth. ZooL Stat. Neap. vi. (1880) pp. 546, G05 f. 



