VISUAL ORGANS OF VERMES. 



155 



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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 

 even in later stages, disappear, 

 or are represented by mere 

 pigment-spots, when they enter upon the fixed mode of life. 



The development of visual organs on the branchial tufts of the 

 head is an adaptation of another kind, which is seen in certain 



Fig. 66. Eye of an Alciopid (Neophauta 

 celox) (after Greeff). i Integument, cover- 

 ing the anterior segment of the bulb, c. 

 I Lens, h Vitreous body, o Optic nerve, 

 o' Expansion of the optic nerve, p Layer of 

 pigment, b Layer of rods. 



