. The chordates. Chordata. Classification of Chordates: Protochordata .599 TENTACLES ENDOSTYLE PERIBRANCHIAL. NERVE CORD ASCIDIA - A UROCHORDATE Fig. 309. Ascidia, a urochordate. The ani- mal is viewed as if cut in sagittal section and seen from the right side. (Redrawn from Sewertzolf, after Boas. Courtesy, Neal and Rand: "Chordate \natomy," Philadelphia, The Blakiston Company.) Of the two external apertures, the more ventral is the inhalant or oral "siphon" and the other the exhalant or atrial ";* The former leads directly to the mouth, which is surrounde
. The chordates. Chordata. Classification of Chordates: Protochordata .599 TENTACLES ENDOSTYLE PERIBRANCHIAL. NERVE CORD ASCIDIA - A UROCHORDATE Fig. 309. Ascidia, a urochordate. The ani- mal is viewed as if cut in sagittal section and seen from the right side. (Redrawn from Sewertzolf, after Boas. Courtesy, Neal and Rand: "Chordate \natomy," Philadelphia, The Blakiston Company.) Of the two external apertures, the more ventral is the inhalant or oral "siphon" and the other the exhalant or atrial ";* The former leads directly to the mouth, which is surrounded by a circle of tentacles. The mouth opens into a greatly enlarged pharynx, which is perforated by numerous gill-slits or stigmas. The action of the cilia on the bars between these slits serves to maintain a current of water from the pharynx into the surrounding peribranchial (atrial) cavity. Along the floor of the pharynx extends a longitudinal grooved thicken- ing, the endostyle, whose surface is ciliated and coated with secreted mucus. A similar groove extends along the dorsal wall of the pharynx, terminating posteriorly at the opening of the esophagus. A circular peripharyngeal ciliated groove joins the anterior ends of the endostyle and the dorsal groove. Particles of food in the water are caught in the mucus and transported along these grooves into the esophagus by ciliary action. Posterior to the pharynx, the alimentary canal consists of a short esophagus, a spherical stomach, and an intestine which leads to an anus opening into the atrial chamber. The heart lies ventral to the esophagus in a coelomic (pericardial) chamber. There are no closed blood-vessels, but the blood is pumped from the heart forward to the pharynx through irregular spaces (lacunas) which resemble functionally the afferent brachial vessels. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these i
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