LA MELLIBRA NCHIA TA. 487 



In Pecten the hypodermis and a thin layer of connective tissue constitute a 

 transparent cornea over the eye, which is situate at the apex of a tentacle. 

 The corneal hypodermic cells form a lens-like body in P. pusio. The 

 hypodermis cells of the surface surrounding the cornea are deeply pig- 

 mented and are termed iris. Beneath the cornea is a mesoblastic cellular 

 lens, convex internally and projecting into a blood-sinus. The optic vesicle, 

 which is derived from an ingrowth of the hypodermis, has its anterior wall 

 converted into a retina, in which the retinophoral rods are turned away 

 from the light as in Vertebrata. Its posterior wall is transformed into 

 a double-layered argentea, in which the cells are obliterated and a red 

 coloured cellular tapetum. Between the argentea and the retinophoral 

 rods is intercalated a thin hyaline vitreous layer secreted by the cells of 

 the argentea (?). The vesicle is invested by a thin mesoblastic sac, which 

 forms a septum between the lens and retina, a sclerotic external to the 

 tapetum. The optic nerve is derived as in all Lamellibranchiata from the 

 circumpallial nerve. It divides into two branches. One which spreads 

 over the sclerotic is central ; its fibres enter the optic vesicle near its rim 

 and become the axial nerves of the retinophorae. The other is lateral ; 

 its fibres pass to the anterior surface of the retina, pierce the septum, 

 and are distributed to the ganglion cells and between the retinophorae. 

 Spondyhts appears to agree closely with Pecten ; but the structure is very 

 much simplified in Cardium 1 . A pair of auditory sacs are usually present 

 when the foot is well developed. They lie close to the pedal ganglion 

 in connection with a nerve from it a . The central cavity contains a single 

 calcareous otolith. 



The mouth is anterior and ventral to the anterior adductor when 

 present. It is elongated transversely, and both its margins, upper and 

 lower, are formed by two prominent ridges which are prolonged laterally 

 into the labial tentacles. These structures are ciliated, and often of great 

 extent, thrown into folds or fringed. The anus is a projecting papilla 

 placed dorsally to the posterior adductor muscle. The alimentary canal 

 is ciliated throughout, and consists of a short oesophagus, a dilated stomach 

 to which is often attached a caecum, and an intestine which is disposed in 

 folds. The terminal section is straight, median, and dorsal, and usually 

 passes through the ventricle, except in Ostrea, Anomia, and Teredo. On 

 the crystalline style, see p. 133, ante. Two ducts open into th stomach, 

 one on either side. They lead to a large branched gland, the liver, which 

 with the genital glands surrounds the coils of the intestine. Calcareous 

 cells are not present in the liver, only granular and ferment cells. The 



1 For the terms used in this description see the note, p. 452, awte\ and for a full account, 

 Patten, op. cit. 



2 This is the usual statement ; but in Anodonta the nerve is said to be derived ffom the 

 cerebro-pedal connective. See p. 138. 



