. Elements of comparative anatomy. Anatomy, Comparative. 351 COMPARATIVE ANATO ]\[Y. simplest tlie eye lies beneath the integument (as in many Opistho- brancliiata). In others it is embedded in the dermo-muscular 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 Mollusca, 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
. Elements of comparative anatomy. Anatomy, Comparative. 351 COMPARATIVE ANATO ]\[Y. simplest tlie eye lies beneath the integument (as in many Opistho- brancliiata). In others it is embedded in the dermo-muscular 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 Mollusca, 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 gradual
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectanatomycomparative
