354 



COMPAEATIVE ANATOMY. 



simplest tlie eye lies beneath the integument (as in many Opistho- 

 brancMata). In others it is embedded in the dormo-musculai' 

 tube^ and retains its superficial position, while at the same time 

 an elongation of the optic nerve is developed. This sub-integu- 

 mentary position must be regarded as a secondary one, for in the 

 MoUusca, just as in the Vermes, the integument takes part in the 

 formation of the eye. The eye-bearing region of the body is 

 ordinarily found to be the base of the tentacle (Prosobranchiata), 

 which may be converted into a special eye-stalk (ommatophor). 

 Or the eye may rest on a process formed from the tentacle 

 (Strombus, Pterocera), or this process may become separated 

 from the tentacle, and become independent. Owing to the pos- 

 session of this optic-stalk the eye is capable of movement; in 

 the Heteropoda this is effected by the muscles that are attached to 

 the wide capsule which encloses the bulb of the eye (Fig, 187, o). 

 Thanks to these muscles the bulb is enabled to vary its position ; in 



form, the bulb is generally 

 rounded or oval; in the 

 Heteropoda it is very pecu- 

 liar (Fig, 187). 



The bulb has a thin outer 

 covering, which passes an- 

 teriorly into the cornea 

 (pellucida), which is formed 

 from the integument. In 

 the posterior portion the 

 optic nerve widens out, and 

 is generally provided with 

 a ganglionic enlargement 

 (r). Internally to it we find 

 the retina, with the end 

 organs of the optic nerve ; 

 these constitute a layer of rods which are turned towards the cavity 

 of the eye, and are separated by a layer of pigment from the 

 external layer of the retina. Behind the cornea there is a lens, which 

 either fills up the cavity of the eye, or has a gelatinous substance 

 posteriorly to it, which represents a vitreous body. 



The sensory layer is formed from the ectoderm, and the lens also 

 is an integumentary structure, inasmuch as it is developed from a 

 cell, which gradually secretes the substance of the lens in stratified 

 layers. 



§ 273, 



The eye of the Cephalopoda resembles, in many points, the optic 

 organ of the Gastropoda. We know that it is gradually differen- 

 tiated from the ectoderm. In Nautilus, each bulb is carried on a kind 

 of optic stalk, and forms a lateral projection (vide supra. Fig. 175, o) ; 

 this is indicated in some of the Dibranchiata, but in them the bulb 

 may be supported by processes of the cephalic cartilage, and lie in 



rig. 187. 



with the 



Upper part of the nervous system, 

 sensory organs of Pterotrachea. 



(IS Cerebral ganglia (Cerebrum), c Commissures. 



Optic capsule. I Lens, ch Pigment layer. 



r Ganglionic enlargement of the optic rerve. 

 a Auditory organ. 



