. Outlines of zoology. Zoology. ALIMENTAR Y S YSTEM. 475 the dorsal and caudal fins, and other skeletal parts occur about the " ; The caudal end of the notochord is quite straight. Nervous system.—The brain has the usual parts, but is small and simple; the roof of the fore-brain is composed of non-nervous epithelium; there is a distinct pineal body, with traces of an eye; the oral part of the hypo- physis is developed from in front of the mouth, and becomes closely connected with the involution of epiblast which forms the nostril. The spinal cord is flattened; the anterior and
. Outlines of zoology. Zoology. ALIMENTAR Y S YSTEM. 475 the dorsal and caudal fins, and other skeletal parts occur about the " ; The caudal end of the notochord is quite straight. Nervous system.—The brain has the usual parts, but is small and simple; the roof of the fore-brain is composed of non-nervous epithelium; there is a distinct pineal body, with traces of an eye; the oral part of the hypo- physis is developed from in front of the mouth, and becomes closely connected with the involution of epiblast which forms the nostril. The spinal cord is flattened; the anterior and posterior roots of the spinal nerves alternate and do not unite; there is no sympathetic fp m Fig. 201.—Longitudinal vertical section of anterior end of larval lamprey.—After Balfour. m., Mouth ; thyroid ; , one of the gill-pouches ; , ven- tral aorta; h., heart; N., notochord; , spinal cord; E., auditory vesicle; ct>., cerebellum; #.!>., pineal body; cerebral hemispheres ; olfactory involution. Though the larva sometimes receives the name of "nine- eyes "—which expresses a popular estimate of the branchial apertures—it is blind, for the eyes are rudimentary and hidden. In the adult they rise to the surface, and are fairly well developed. The optic nerves do not cross, as they usually do. The ear has only two semicircular canals instead of the usual three. The single nasal sac does not open posteriorly into the mouth as it does in Myxine; though prolonged backwards it ends blindly. Its external .opening is at first ventral, but is shunted dorsally and Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Thomson, J. Arthur (John Arthur), 1861-1933. Edinburgh, Glasgow and London, H. Frowde and Hodder & Stoughton
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Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
