240 THE LAMELLIBRANCHIA 
tapetum—of cuticular or secretory origin, which gives the eyes of 
Pecten and Spondylus their brilliant lustre. The optic nerve arises 
from the circumpallial nerve and subdivides: one of its branches 
passes round the ocular globe to reach the retina. Between the eye 
and the external corneal epithelium or pellucida there is a cellular 
lens or conjunctiva, which is extra-ocular and consequently sub- 
epithelial. In some cases the corneal epithelium itself is thickened 
above the eye (Fig. 217, co). 
In some species of Cardiwm—in C. rusticum, for example—the 
siphons are the only parts of the animal which project from the 
bottom when the animal is buried, and the tentacles surrounding 
them are provided with eyes whose structure is analogous to that of 
the eyes of Pecten and Spondylus, with this difference, that in the 
former the pigment is situated in the connective tissue surrounding 
the ocular globe. 
5. Generative Organs.—The sexes are separate in the Lamelli- 
branchia in general, but the whole order of Anatinacea is herma- 
phrodite, and also some small isolated groups, viz. the Cyrenidae, the 
genera Poromya, Tridacna, Kellya, Lasaea, Entovalva, and Scioberetia 
(the two last named being parasitic), and certain species of Pecten, 
Ostraea and Cardiwm, and Anodonta imbecilis. Sexual dimorphism is 
recognisable only in certain species of Unio (U. batavus and U. tumidus) 
and in Lampsilis, in which the female is rather broader than the male, 
and in Astarte, in which the border of the shell is smooth in the male 
but crenelated in the female. In the genus Yeredo there is hyper- 
polygyny, the males being only in the proportion of 1: 500 to the 
females. There is never a copulatory organ, nor yet an accessory 
gland, unless perhaps in the male Cuspidaria. The gonads are 
paired and symmetrical, superficially placed, and generally occupy 
the most dorsal and posterior part of the visceral mass, often extend- 
ing thence into the foot. They are united and communicate with 
one another in Donax, Lasaea, Adacnarca, Chlamydoconcha, Cuspidaria, 
etc. In exceptional cases they extend into the mantle, either into 
both lobes, as in all the Mytilidae except Dacrydium and some 
species of Chama, or into one lobe only as in the Anomiidae. In 
some genera of Lucinidae, e.g. Montacuta and Azinus, the gonads, 
together with parts of the liver lobes, project into the pallial cavity 
in the form of arborescences. Each gonad is an acinous structure 
and its caeca may be much ramified, for instance, in Ostraea. 
In the most primitive arrangement there is no proper generative 
aperture ; each gonad discharges its products into the reno-peri- 
cardial duct, as may be seen in the Protobranchia (Solenomya, ete.) ; 
but a secondary union between the reno-pericardial duct and the 
external extremity of the postero-anterior branch of the kidney 
allows the generative products to pass direct to the external renal 
orifice (Fig. 213, IV, II). In many other forms the gonad still opens 
