From near the entrance of the optic nerve, through the original choroidal fissure, arises the much-folded pecten, deeply pigmented and very vascular, far into the vitreous humour.
Hickson and others, that in the bivalves Pecten and Spondylus, which also have eyes upon the mantle quite distinct from typical cephalic eyes, there is the same relationship as in Oncidiidae of the optic nerve to the retinal cells.
In both Oncidiidae and Pecten the pallial eyes have probably been developed by the modification of tentacles, such as coexist in an unmodified form with the eyes.
The praegenital somite, VII PrG, is still present, but has lost its rudimentary appendages; go, the genital operculum, left half; Km, the left pecten; abp 4 to abp 7, the rudimentary appendages of the lung-sacs.
In some Lamellibranchs (Pecten, Spondylus, Pholas, Mactra, Tellina, Pectunculus, Galeomma, &c.), although cephalic eyes are generally absent, special eyes are developed on the free margin of the mantle-skirt, apparently by the modification of tentacles commonly found there.
A very few have the power of swimming by opening and shutting the valves of the shell (Pecten, Lima); most can crawl slowly or burrow rapidly; others are, when adult, permanently fixed to stones or rocks either by the shell or the byssus.
Such eyes on the mantle-edge are found in Pecten, Spondylus, Lima, Pinna, Pectunculus, Modiola, Cardium, Tellina, Mactra, Venus, Solen, Pholas and Galeomma.
The glochidium quits the gillpouch of its parent and swims by alternate opening and shutting of the valves of its shell, as do adult Pecten and Lima, trailing at the same time a long byssus thread.
Pecten; shell orbicular, with equal auriculae; without a byssal sinus; British.
The pallial eyes of Pecten and other Lamellibranchs, and of Chitons.
That the level of Lake Aral was much higher in postPliocene times is proved by the discovery of shells of its characteristic species of Pecten and Mytilus in the Kara-kum Desert, 33 m.
Among other well-known Jurassic genera Nathorst has identified the following: Equisetites, Cladophlebis, Todites, Thinnfeldia, Otozamites, Williamsonia pecten, Araucarites.
16), and others known under such names as Pterophyllum or Ptelophyllum pecten, &c., both common Jurassic species - as well as to stems bearing peduncles with terminal oval flowers, similar in form to those of Bennettites.