266 Introduction to Animal Morphology. 



geneous basement membrane lined by ciliated epithe- 

 lium, and filled with fluid, holding one or more glo- 

 bular otoliths, which during life rotate rapidly ; they 

 have either one nerve in each (Unio), or else they lie 

 directly on the ganglion ; sometimes they are deeply 

 imbedded in the foot (Cytherea) ; 4th, eyes exist in 

 the larvae of all as pigment spots with a lens, seated 

 on the head under the velum ; these in the adult are 

 lost, but secondary eyes develop as pigment specks, 

 with neither nerves nor crystal cones, as the siphonal 

 eyes of Solen and Mactra, or else as true eyes along the 

 mantle edge, sessile or stalked, consisting of cornea^ 

 lens, crystal cones imbedded in pigment, iris with a 

 movable pupil, and in Pecten and Spondylus there is 

 a tapetum cellulosum. Into each eye in Pecten' two 

 branches from the circumpallial nerve enter, one into 

 the fundus, and one into its side ; these eyes may be 

 from 8 to 90 in number, and are often brightly 

 colored. The senses and psychic phenomena in la- 

 mellibranchs are of a low order, but a degree of me- 

 mory is evinced if it be true that oysters taken from 

 beds occasionally uncovered at low water never open 

 their shells, while those taken from greater depths are 

 more careless about the opening and closure of their 

 valves. 



Lamellibranchs have symmetrical, usually dioecious 

 sexual organs, with no accessory apparatus ; some 

 (Ostrea, Pisidium, Pandora, Kellia, Galeomma, Cyclas, 

 &c.), are monoecious. In Ostrea the ovigerous and 

 spermigerous acini are mixed side by side in the one 

 gland.* In Pecten varius the gland is bilobed, the 



* But according to Davaine they are not functional together, but pro- 

 tandrous, the male acini reaching perfection when the females are unripe. 



