422 BIOLOGY. [BooK vi. 



The eyes of the asterise are situated at the extremity of the 

 radii. They are spherical bodies reposing on a nervous mass. 

 They are surrounded by an envelopment of pigment, and covered 

 with an epithelial cuticle. 



Visual organs are met with in the mollusks at all degrees of 

 development. They are absent in the fixed mollusks. Certain 

 of these last, which in the state of mobile larvae had eyes, lose 

 them by organic degradation when they have become immobile. 

 Certain species of lamellibranchians have as organs of vision 

 sometimes pigmentary spots, sometimes brilliant organs, dissemi- 

 nated on the edge of the mantle and receiving nerves. 



The eyes of the cephalopods and cephalophores are always two 

 in number, and placed usually at the base of the tentacles, 

 sometimes at the extremity of special tentacles. They are more 

 over very irregularly developed. Sometimes they are simple 

 pigmentary spots situated on the cesophagian ganglion; some- 

 times, for example, they are complex organs, comparable with 

 the eyes of the vertebrates. They then rest on a ganglionary 

 nervous expansion, have a retina, a layer of optical bacilla, 

 separated from the deep layer of the retina by a pigmentary 

 stratum. On this retina rests a sort of vitreous body, in front 

 of which is a lens, covered by a cornea. A thin integument 

 covers all the apparatus. The eye of the sepia is constructed 

 on this plan, but with great perfection (Fig. 79). * 



In sum, the complete eye is composed first of all of optical 

 bacillaria, that is to say of small special organs, destined to 

 receive, or rather to select the luminous waves, and to transmit 

 the molecular agitation to the optical nervous threads. That is 

 the essential part of the organ of vision. In advance of these 

 bacillaria, refractive media concentrate the luminous rays, and 

 thus intensify the impression. Finally, a pigmentary envelop- 

 ment isolates, more or less perfectly, the sensitive elements. 



How remarkable soever the compound eyes of many arthro- 

 pods may at first sight appear, these organs do not differ 

 essentially from those of the other zoological groups. The eye 



