. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. 88 ANATOMY OF AMPHIOXUS. WijHE that the inner plexus on both right and left halves of the oral hood is exclusively formed by nerves which arise from the left side of the central nervous system ; and, further, that the nerve-supply of the velum is fur- nished by branches from the fourth, fifth, and sixth dorsal nerves of the left side only. This asymmetrical innerva- tion of the velum and inner (glandular) surface of the oral hood will be referred to again after the consideration of the larval development. Th
. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. 88 ANATOMY OF AMPHIOXUS. WijHE that the inner plexus on both right and left halves of the oral hood is exclusively formed by nerves which arise from the left side of the central nervous system ; and, further, that the nerve-supply of the velum is fur- nished by branches from the fourth, fifth, and sixth dorsal nerves of the left side only. This asymmetrical innerva- tion of the velum and inner (glandular) surface of the oral hood will be referred to again after the consideration of the larval development. The peripheral ganglionic enlargements which are so characteristic of the two pairs of cranial nerves must be cor- related with the sensibility of the snout. As the nerve-fibres are continued beyond them, they are not to be regarded as end-organs, but simply as peri- pheral ganglia. Their structure is shown in Fig. 44. They were discovered by the great French naturalist Ouatrefages in 1845. Each of them is composed of from one to four nerve-cells, with granular protoplasm and a large nucleus. Each group is enclosed in a sheath which is a continuation of the sheath of the nerve itself. The sheath is lined internally by an endo- thelium. According to Fusari the nerve-fibres enter into direct connexion with the cells, though some would appear to pass round them. The peripheral nervous system of Amphioxus can only be compared definitely, at present, in its broadest features with that of the higher Vertebrates. The determination. Fig. 44. — Peripheral ganglion cells of the cranial nerves of Amphi^ oxus. (After Fusari.). 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 Willey, Arthur, 1867-. New York, London, Macmillan
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