. Elements of comparative anatomy. Anatomy, Comparative. Fig. 65. Head and most anterior segments of a Myria nida. a Eyes, b Tentacles, c Unpaired cephalic tentacle, d Cirri. Among the Aimclidos we find the eyes of the Chastopoda gene- rally hidden beneath the integument, and placed on the cerebral ganglion in one or two pairs : a single eye is seldom present. Generally one pair is consider- ably developed, and the second often reduced to a pigment-spot. Where these visual organs are specially developed they stand out on the surface of the integument (Sylluke, Nereida), Fig. 65, a); and may at
. Elements of comparative anatomy. Anatomy, Comparative. Fig. 65. Head and most anterior segments of a Myria nida. a Eyes, b Tentacles, c Unpaired cephalic tentacle, d Cirri. Among the Aimclidos we find the eyes of the Chastopoda gene- rally hidden beneath the integument, and placed on the cerebral ganglion in one or two pairs : a single eye is seldom present. Generally one pair is consider- ably developed, and the second often reduced to a pigment-spot. Where these visual organs are specially developed they stand out on the surface of the integument (Sylluke, Nereida), Fig. 65, a); and may attain to a highly compli- cated structure. This is the case in the Alcioptc, the pelagic mode of life in which is in con- nection with the high grade of development of this sensory organ. This influence of the mode of life is also seen in their nearest allies, the Phyllodoceida3, which live at the bottom of the sea, and have rudimentary or very simple eyes. The spherical bulb (Fig. (36) presents this, the highest degree of development in the Alciopidas only. The integument (c) covers the anterior, strongly-curved seg- ment, immediately behind which there is a spherical lens (/). The hinder segment, the innermost layer of which forms the layer of rods (b), surrounds a homogeneous vitreous body (li). A layer of pigment (p) separates the layer of rods from the parts of the retina which lie more to the exterior; outside all these is the expansion of fibres of the optic nerve (o'). While in the simpler forms of eye the ter- minal organs of the nerve lie in the integument, they are here pressed together into a concave layer. Influential in the development of this arrange- ment is the multiplication of the perceptive elements, and the formation of refracting media. Just as the eyes are completely wanting in the ma- jority of the Scoleina which live in the dark, so also these organs undergo degeneration in the Tubicola among the Chastopoda. The eyes which are present in the larvas, and
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectanatomycomparative
