2 4 o 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 Cardium 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 Cardium, and Anodonta imbedlis. 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 Teredo 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 Axinus, 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, etc.) ; 

 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 



