ON THE EYES OF SOME INVERTEBEATA. 679 



to these cells or perhaps to the retinulse, or if they are homo- 

 logous with the pigment-cells. 



The cone penetrates with its point deeply into the retinula. 

 The latter surrounds a thin rhabdom which is probably four- 

 edged. 



The ganglion lies in front of the brain ganglion, separated 

 from it only slightly by a narrow furrow. Its rind consists in 

 the front and at the sides only of a thin, at the back of a 

 thicker, layer of nuclei through which the nerve-fibres enter 

 from the brain-ganglion into the ganglion opticum. The 

 medullary substance of the latter can be separated into a front 

 and a back segment. 



The outgoing nerve-fibres pass as a spindle-shaped string 

 into the centre of the eye, where they touch the ends of the 

 retinulse. The string is without pigment until it subsides 

 between the crystal cones, and is surrounded by a pigment 

 envelope between the retinulse. 



With the exception of the entrance of the nerves, which 

 occupies the space of about one or two eye-units, the eye-units 

 radiate equally towards all directions, both forwards, sidewards, 

 and backwards, being directed to the translucent body. 



Just as the camera obscura eye is common to the vertebrates 

 and the molluscs, so there are also met with in the molluscs 

 organs of sight of the type of the fan-eyes. 



For a long time past eyes have been attributed to the 

 Lamellibranchiata. If these animals possess such organs they 

 can only lie on two places of the body : at the mantle border 

 or on the siphon. At the latter place modern zoologists have 

 not been able to find any organs of sight, whilst at the 

 mantle border are situated, in many species, organs of different 

 form and composition, which are considered partly wrongly 

 (Pinna) partly rightly as eyes. 



Their construction is the simplest in Area and Pectun- 

 culus. In Pectunculus glycimeris there occur on the 

 edge of the interior mantle fold small spherical dark-brown 



