. The chordates. Chordata. 402 Comparative Morphology of Chordates AT5OP0RE. Fk;. 312. Amphioxus, in ventral and side views. Metamerism, lacking in mo- chordates, and scarcely evident in liemichordates, is strikingly shown hy Amphi- oxus. \\ liether this metamerism is inherited from annelid-like ancestors or is a con- vergent trait independently acquired, is problematic. (After Kirkaldy. Courtesy, Neal and Rand: "Chordate Anatomy," Philadelphia, The Blakiston Company.) that of urochordates. A much-enlarged and elongated pharynx com- municates, through numerous (over 100) pai
. The chordates. Chordata. 402 Comparative Morphology of Chordates AT5OP0RE. Fk;. 312. Amphioxus, in ventral and side views. Metamerism, lacking in mo- chordates, and scarcely evident in liemichordates, is strikingly shown hy Amphi- oxus. \\ liether this metamerism is inherited from annelid-like ancestors or is a con- vergent trait independently acquired, is problematic. (After Kirkaldy. Courtesy, Neal and Rand: "Chordate Anatomy," Philadelphia, The Blakiston Company.) that of urochordates. A much-enlarged and elongated pharynx com- municates, through numerous (over 100) pairs of gill-openings, with a peribranchial (atrial) cavity which has exit to the exterior via a median ventral atriopore just behind the elongated pharyngeal region. In contrast to the other protochordates, in which there is no segmenta- tion of the body, the body-muscle of cephalochordates is definitely segmented (about 60 pairs of myomeres in Amphioxus) throughout the whole length of the animal, and the nerves which emerge from the nerve-cord, like the spinal nerves of a vertebrate, are arranged segmen- tally to correspond to the myomeres. In further contrast to other proto- chordates, Amphioxus has numerous (about 90) pairs of excretory or renal tubules, but they are not aggregated into definite kidneys. The embryonic origin of these tubules has been disputed. If, as now com- monly held, they are derived from the ectoderm of the embryo, they would seem to correspond to the excretory nephridia of annelid worms rather than to the kidney-tubules of vertebrates, which develop from the mesoderm. The skin of Amphioxus is a simple-epithelial epidermis which se- cretes a delicate external protective cuticula (Fig. 11 A). The ventral mouth, surrounded by a circle of tentacles, leads directly into the pharynx (Fig. 313). A median ventral groove whose surface is mucous and ciliated extends the entire length of the pharynx, constituting an endostyle similar to that of urochordates. In th
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