. L'histoire naturelle de Cambridge. Zoologie. 67 Structure de ganglion. Pas de glande neurale est trouvé. L'ouverture branchiale ou bouche mène à la simple sac branchial ou du pharynx (Fig. 30, hr.s). Il n'y a pas de tentacules. L'endostyle est courte, est un tube fermé à la fois l'avant et à l'arrière (Fig. 29), et dispose d'environ quatre rangées longitudinales de cellules glandulaires. Il n'y a pas de lame dorsale, et les bandes péripharyngienne exécuter dorsalement et posteriorly pour unir près en face de l'ouverture de l'œsophage. Le mur de la sac branchial ne montre pas la structure complexe d'habitude en Tuni- cata, et n'a que deux ap cilié

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. The Cambridge natural history. Zoology. STRUCTURE 67 ganglion. No neural gland is found. The branchial aperture or mouth leads into the simple branchial sac or pharynx (Fig. 30, hr.s). There are no tentacles. The endostyle is short, is a closed tube both anteriorly and posteriorly (Fig. 29), and has about four longitudinal rows of gland-cells. There is no dorsal lamina, and the peripharyngeal bands run dorsally and posteriorly to unite close in front of the oesophageal opening. The wall of the branchial sac does not show the complex structure usual in Tuni- cata, and has only two ciliated apertures (Figs. 30, 31, 32, sg). These are homologous with the primary stigmata of the typical Ascidians, and with a pair of the gill-clefts of Vertebrates. They are placed far back on the ventral surface, one on each side of the middle line, and lead into short funnel-shaped tubes which open on the surface of the body behind the anus (Fig. 30, at). These tubes corre- spond to the right and left atrial in- FlO-29.—Transverse section through anterior part of Oiio volutions, which in an ordinary Asci- dian fuse to form the peribranchial cavity. The remainder of the alimentary canal consists of oesophagus, stomach (which may have a glandular diverticulum), intestine and rectum (Fig. 30). The heart, surrounded ventrally by a delicate pericardial membrane, lies below and in front of the stomach, and is formed by the differentiation of the outer ends of epithelial cells into muscular fibrillae. Two specially large glandular cells are placed at the opposite ends of the heart. There are no blood-vessels except the remains of the primary body-cavity (blastocoel). No heart can be seen in some of the smaller species of OiJcopleura. Nearly all the species are herma- phrodite, and the large ovary and testis are placed at the posterior end of the body. There is no proper oviduct, the genital pro-. pleura to show ganglion, sense-organs, endostyle, etc. X 300. ir.s. Branchial sac ; c.f, cilia

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• bookcentury1800
• bookdecade1890
• booksubjectzoology
• bookyear1895