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The comparative morphology and evolution of the eyes of caecilians

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Zoomorphology (1985) 105:277-295 Zoomorphology © Springer-Verlag 1985 The comparative morphology and evolution of the eyes of caecilians (Amphibia, Gymnophiona) Marvalee H. Wake Department of Zoology and Museum of Vertebrate Zoology, University of California, Berkeley, CA 94720, USA Summary. Caecilians (Amphibia, Gymnophiona) have been reported to have 'vestigial' eyes, to lack some or all of the extrinsic eye muscles and their nerves, and to utilize eye muscles and glands, or derivatives of them, to effect movement of the tentacle, a chemosensory structure unique among vertebrates. Morphological evidence indicates that the eye is a functional photoreceptor in virtually all species examined, with an intact retina and optic nerve. The pattern of retention of extrinsic muscles varies. The ontogeny of the eye of Dermophis mexicanus is typical of that of most vertebrates, though components of accommodation never develop. Several taxa are reported in the literature to lack various eye structures; the present study reveals them to be variously present. Evolutionary trends in caecilian eye morphology include the following: (1) the eye is overlain by thicker, offen glandular skin, to overlain by bone as weil as skin; (2) extrinsic muscles become attenuate, and some to all may be lost; (3) the retina has the typical verte- brate layered organization, to having a reduced cell number, to becoming net-like rather than stratal; (4) the optic nerve is present, becoming attenuate, perhaps represented only by glial cells; (5) the lens is round (aquatic forms, larval and adult) to spheroid; lens crystalline to cellular (retention of the embryonic condition) to amorphous to absent; (6) the vitreous body is reduced or lost; (7) the cornea adheres to the overlying dermis or periosteum; the lens is free to adherent to cornea to adherent to both cornea and retina. Scolecomorphids have the eye pulled out of the socket and embedded in the tentacle under the skin of the upper jaw. This pattern of trends in eye reduction is similar to that observed in other vertebrate lineages that are fossorial or troglobitic. A. Introducüon Caecilians are structurally modified for a fossorial existence - they are elongate, limbless, and have flattened, heavily ossified skulls. Their eyes are small, and covered by skin or skin and bone. Most caecilians upon which field observa- tions have been made are nocturnal foragers (Taylor 1968; Wake pers. obs.). Further, caecilians have a highly elabor- ated chemosensory system, utilizing both narial olfaction and the tentacle, a structure unique to caecilians among vertebrates, whose lumen is continuous with the Jacobson's organ. It has long been known that the tentacle coopts eye muscles, nerves, and glands during its development and subsequent function (Leydig 1868 ; Wiedersheim 1879; P. and F. Sarasin 1887-1890). The retractor bulbi, innervated by the abducens nerve, becomes the retractor of the tenta- cle; an internal rectus muscle, innervated by the oculomotor nerve, is often the retractor of the tentacle sheath; the leva- tor bulbi, innervated by a branch of the ramus mandibularis of the trigeminal, becomes the compressor muscle of the glands surrounding the tentacle; and the Harderian gland, which lubricates the orbit in most terrestrial vertebrates, moistens the tentacular channel. Because of the small size of the eye, its covering layers, the loss tO the tentacle of structures associated with the eye in other vertebrates, and the fossorial or swimming and nocturnal habitus of the animals, many workers have considered the caecilian eye "rudimentary", "vestigial", and/or "degenerate" (Norris and Hughes 1918; Walls 1942) despite, or because of, their own investigations of caecilian cranial structure. Work on various species suggests a mosaic pattern of loss of extrinsic eye muscles and their nerves, and their utilization by the tentacle, and of the functional properties of the eye. For example, Norris (1917) and Norris and Hughes (1918) considered the eyes of all caecilians rudimen- tary, but in those species with the eye covered by skin, they thought that the eye probably was affected by light. They stated that such forms have some or all of the eye muscles and nerves present, but mostly vestigial or modified for use by the tentacle. The other group that Norris recog- nized included the species in which the eye is covered by bone (maxillary and/or squamosal, rarely frontal) as weil as by skin, and Norris and Hughes declared that "the optic nerve and all the eye-muscles and eye-muscle nerves, except the retractor bulbi and its nerve, the abducens, have com- pletely disappeared". I infer that Norris considered such eyes non-functional as photoreceptors. However, the litera- ture is in conflict about the presence or absence of certain eye structures both within and among species. Therefore, as part of a larger study of the developmental and func- tional morphology of the head of caecilians, I have under- taken an examination of the eye and tentacle of a number of species in several families. I here present information on the comparative morphology and development and function of the eye. I offer a new interpretation of the struc- ture of the eye and its variation, and consider the morpho- logical evidence that allows interpretation of function.278 Table 1. Heads sectioned for study Taxon Total length (mm) Developmental stage Ichthyophiidae lchthyophis sp. 11 l Ichthyophis glutinosus 344 Ichthyophis orthoplieatus 380 Uraeotyphlus narayani 151 Caeciliidae Afrocaecilia taitana 246 Boulengerula boulengeri 171 Caecilia occidentalis 510, 489 Dermophis mexicanus 5-398 Geotrypetes grandisonae 101 Geotrypetes seraphini 15-335 Gymnopis multiplicata 200, 370 Hypogeophis rostratus 5-224 ldiocranium russeli 94 Oscaecilia ochrocephala 432 Schistornetopum thomense 224 Scolecomorphidae Scolecomorphus kirkii 275 Scolecomorphus ulunguruensis 271 Typhlonectidae Typhlonectes compressicaudus 15-350 Larva Adult Adult Adult Adult Adult Juvenile 10 embryos, 14 fetuses, 6 juveniles, 7 adults Larva 1 embryo, 2 juveniles, 2 adults 1


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