82 PATTEN. [Vol. I. 



consists of a roughly spherical mass of large cells, which emit, 

 when living, a faint red color, reflected by the underlying 

 argentea. I have counted, with a pocket lens, fifty-one eye- 

 bearing tentacles on one individual. 



The extremely simple retina, which is oblong in shape, — the 

 short diameter being at right angles to the pigmented covering, 

 — consists of five or six rows of cells, the ends of which, being 

 directed inwards, rest upon the mass of connective-tissue fibres 

 which serves at once as a capsule and tapetum. The opposite 

 extremities of the retinal cells, where the large oval and sharply 

 stained nuclei are situated, appear to terminate in single nerve- 

 fibres, which pass out of the capsule and, bending at right 

 angles, extend along the axis of the tentacle. At the angle of 

 each of these cells, nearly opposite the large nuclei, is a small 

 and poorly defined cell containing a minute but deeply stained 

 nucleus. 



The argentea is similar to that of P^^r/^w, and consists of nucle- 

 ated connective-tissue cells, the bodies of which are flattened 

 into membranes, composed of minute refractive squares. The 

 argentea envelops the whole eye, but is thickest on the sides 

 next the pigment, and toward the base of the tentacle. Whether 

 the inner ends of the retinal cells are provided with rods similar 

 to those of Pecten could not be determined with certainty. 



The round cellular body situated in front of the retina is com- 

 posed of large, characteristic cells, which, however, are not con- 

 fined to this region alone, but extend thence, in a double row, 

 nearly half the length of the tentacle. 



In Cardium tuberculatum the tentacles are also provided with 

 eyes, although the pigmented patches at the tips of the tenta- 

 cles are absent. 



In Cardita sulcata isolated ommatidia are present, but no 

 tentacular eyes. 



The most important fact obtained by studying the eyes of 

 Haliotis was that the colorless cells are not gland-cells or of 

 secondary importance to the retina, but are true retinophorae, 

 having double rods, two nuclei, and an axial nerv^e fibre. The 

 pigmented cells, or retinulae, are provided with single, club- 

 shaped rods, which contain a net-work of fibrillae formed by the 

 ramification of intercellular nerve-fibres. The most satisfactory 



