THE EYES OF BIVALVED SHELLFISH 155 



Cephalopods the eyes may be highly complex, as in Sepia, having a retina 

 of the upright type, a vitreous chamber, a biconvex double lens, " ciliary 

 body," " iris," anterior chamber, cornea, sclerotic cartilages, ocular 

 muscles, and palpebral folds, or a simple unclosed vesicular pit, open to the 

 surface and thus containing sea-water as in Nautilus. An inverted type 

 of retina is, moreover, present in Onchidium (Fig. 39, Chap. 3, p. 54), an 

 aberrant type of snail in which eyes are present on the back of the animal 

 and the nerve fibres lie in front of the retinal cells and rods ; the lens is 

 formed of two (or more) enormous clear cells which fill the whole of the 

 interior of the eye, occupying the space which in a vertebrate eye would 

 be filled by the vitreous humour and lens. The resemblance to a verte- 

 brate eye is further simulated by the way in which the fibres of the optic 

 nerve appear to perforate the retina at the posterior pole of the eyeball. 



The Eyes of Bivalved Shellfish (Pelycypoda) 



In the freshwater mussels (Anodonta or Unio) and cockles (Cardium) no 

 eyes are present in the adult animal and none have been described in the 

 larval stage, although a typical trochophore larva is developed with an 

 apical plate bearing a vertical tuft of hairs beneath which is a cerebral 

 ganglion, while surrounding the plate is a prototroch or girdle of ciliated 

 cells, as in Patella (Fig. 114). 



In the scallop, or Pecten, Fig. 106, eyes are present round the edge of 

 the mantle. These are peculiar in having an inverted retina and a cellular 

 lens . They probably originate as a special modification of certain tentacles . 

 Each eye forms a dome-shaped projection about the size of a pin's-head. 

 Enclosed within this is an almost spherical vesicle (Fig. 107) the inner half 

 of which is formed by the retina, while the outer includes the cornea and 

 lens. The retina is slightly concave in front where it comes in contact 

 with the lens and is in relation with a circular " blood sinus." It is 

 covered by a basement membrane beneath which is a nerve-fibre layer. 

 The nerve-fibres on one side converge towards a point on the edge of the 

 cup where, joining together, they form one branch of the optic nerve. 

 Other fibres diverge as they pass towards the edge of the cup. On reaching 

 the outer margin of the cup they turn round this, and passing backward 

 on the superficial surface of the sclera converge towards the " posterior " or 

 inner pole of the eye, where they unite to form a second branch of the optic 

 nerve. The two branches join a short distance behind the posterior pole to 

 form a single main optic nerve. Beneath the layer of nerve-fibres is a 

 stratum of large nerve cells which appear to give origin to the optic nerve- 

 fibres, these fibres springing from the superficial end of the cell, namely 

 that nearest the lens. Deep to these large cells is a plexiform layer con- 

 taining small nuclei. This is succeeded by a layer of refractile rods which 



