. Elementary text-book of zoology. ASCIDIA. 289 An incision will reveal at once that the body of the animal is enveloped in a thick test (or tunic), out of which Internal "^^ animal may be removed entire, like a bean . out of its pod. 1 he test is thick and of a semi - transparent, gelatinous appearance. It is produced chiefly by the layer of underlying ectoderm, Fig. 203.—Diagrammatic Median Longitudinal Section THROUGH an AsCIDIAN. Mouth. Buccal Cavity. Atriopore. Endostyle.. Pharyngeal part of Ventral Vessel. Heart. "" Gonad. Stomach. Intestinal part of Ventral Vessel. The

. Elementary text-book of zoology. ASCIDIA. 289 An incision will reveal at once that the body of the animal is enveloped in a thick test (or tunic), out of which Internal "^^ animal may be removed entire, like a bean _ . out of its pod. 1 he test is thick and of a semi - transparent, gelatinous appearance. It is produced chiefly by the layer of underlying ectoderm, Fig. 203.—Diagrammatic Median Longitudinal Section THROUGH an AsCIDIAN. Mouth. Buccal Cavity. Atriopore. Endostyle.. Pharyngeal part of Ventral Vessel. Heart. "" Gonad. Stomach. Intestinal part of Ventral Vessel. The Stock Photo

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. Elementary text-book of zoology. ASCIDIA. 289 An incision will reveal at once that the body of the animal is enveloped in a thick test (or tunic), out of which Internal "^^ animal may be removed entire, like a bean _ . out of its pod. 1 he test is thick and of a semi - transparent, gelatinous appearance. It is produced chiefly by the layer of underlying ectoderm, Fig. 203.—Diagrammatic Median Longitudinal Section THROUGH an AsCIDIAN. Mouth. Buccal Cavity. Atriopore. Endostyle.. Pharyngeal part of Ventral Vessel. Heart. "" Gonad. Stomach. Intestinal part of Ventral Vessel. The right half has heen removed. and^consists of a hyaline basis containing cellulose (a material mainly confined to the plant-kingdom), through which are scattered a number of cells. Below the. 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 work.. Masterman, Arthur Thomas. Edinburgh, E. & S. Livingstone

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RMRDH067–. Elementary text-book of zoology. 294 CHORD AT A. presence of the atrium, Ascidia can be directly compared with the other class of the Atriozoa. The conclusions drawn from these characters have, how- ever, an ample corroboration in the development. The eggs are laid into the atrium, in which they are fertilised and pass their early stages. Later, the larva is free-swimming and pelagic. The segmentation is total and nearly equal, producing a blastula which is invaginated to form a gastrula. The gastrula elongates, and the blastopore comes to lie in a postero-dorsal position in relation to the

RMRJNAHA–. Elementary text-book of zoology [electronic resource]. Zoology. ASCIDIA. 293 Fig. 202.—Oblique Section through an Ascidian. {Ad. not.) Brain.. presence of the atrium, Ascidia can be directly compared with the other class of the Atriozoa. The conclusions drawn from these characters have, how- ever, an ample corroboration in the development. The eggs are laid into the atrium, in which they are fertilised and pass their early stages. Later, the larva is free-swimming and pelagic. The segmentation is total and nearly equal, producing a bias tula which is invaginated to form a gastrula. The gastr

RMRDH07B–. Elementary text-book of zoology. ASCIDIA. 293 Asddia is hermaphrodite. The testes and ovaries are . ,^, simple paired sacs lying over the stomach KeproductlTe. 1 1 1 • i t • 1 and leading by separate ducts into the atrium. The main interest attaching to Ascidia is involved in its development. If the anatomical account has been carefully Fig. 205.—Development of an Ascidian. (Afler KOWALEVSKI.) Blastula, Gastrula. Blastocoele. Hypoblast. Blastopore.. Neuropore. Neural Tube. The various stages are shown in median section. followed, it will have been noticed that Ascidia differs from Developm

RMRJNAA7–. Elementary text-book of zoology [electronic resource]. Zoology. AMPHIOXUS. 299 Fig. 211.—View OF Amphioxus from THE Right Side. {Ad. nat.) Dorsal Fin Skeleton Notochoid. resembles the pharynx of Ascidia. Its internal walls are mostly ciliated. The endostyle extends along the median ven- tral line, joined by peripharyngeal bands to a median dorsal epi- branchial groove. The lateral walls of the pharynx are perforated by a great number of pharyngeal clefts which run diagonally back- wards as long slits. These pharyngeal clefts are twice as numerous as those of the larva, each of the latter

RMRJNA98–. Elementary text-book of zoology [electronic resource]. Zoology. 300 CHORD ATA. oxus, however, the atrium is not continued round the dorsal Hne of the pharynx. In Ascidia it was the mid- ventral portion which was incomplete. The atrium is continued backwards behind the pharynx and along the intestine until it terminates in the atriopore, through which the water has exit. Lying on the right side of the pharynx in the atrium is a long hollow sac, the liver, which opens into the alimentary canal at the junction of pharynx and intestine. The intes- tine is produced backwards as a long tube to the

RMRJNAEX–. Elementary text-book of zoology [electronic resource]. Zoology. ASCIDIA. 295 Fig. 205.—Transverse Section through Embro ck an ASCIDIAN. (After Delage.) Neural Groove. Notochord, Ep:blast.. Archenteron. Fig. 206.—Chordula Larva of an Ascidian. (After KowALEVSKi.) er.ropore. hypoblast, and a pair of lateral mesoblastic masses mere or less broken up. T his larva is characteristic of the Chordata though only found as a larva in the Atrtozoa, being represented by an embryonic stage in Vertebrata. The further development of the ascidian diverges from that of the next class. The larva becomes divi

RMRDH021–. Elementary text-book of zoology. AMPHIOXUS. 299 Oral Cirri. resembles the pharynx Fig. 213.—View of Amphioxus from of Ascidia. Its internal the Right Side. {Ad nat.) walls are mostly ciliated. The endostyle extends along the median ven- tral line, joined by peripharyngeal bands to a median dorsal epi- branchial groove. The lateral walls of the pharynx are perforated by a great number of DorsaiFin pharyrigeal clefts which run diagonally back- wards as long slits. These pharyngeal clefts are twice as numerous as those of the larva, Notocnord each of the latter be- coming divided longi- u tudin

RMRJNAK4–. Elementary text-book of zoology [electronic resource]. Zoology. ASCiDIA. 2^9 An incision will reveal at once that the body of the animal is enveloped in a thick test (or imiic), out of ^hich Internal animal may be removed entire, like a bean Features P°^' ^^^^ thick, and of a semi - transparent gelatinous appearance. It is produced chiefly by the layer of underlying ectoderm Yg. 20I.—Diagrammatic Median Longitudinal Section THROUGH AN AsCIDIAN. Mouth. Atriopore.. Buccal Cavity. Peri-Pharyn- geal Groove. Pharyngeal -V Clefts. Mantle (Ectoderm), Endostyle, Pharyngeal part of Ventral Ves'iel.

RMRDH05E–. Elementary text-book of zoology. ASCIDIA. 29s Fig. 207.- -Transverse Section of Larva of Ascidian. (After Van Beneden and Julin.) Nerve Cord. Mesoblastic Sac.. Notochord. Hypoblast. hypoblast, and a pair of lateral mesoblastic masses more or less broken up. This larva is characteristic of the Chordata though only found as a larva in the Atriozoa, being represented by an embryonic stage in Vertebrata. The further development of the Ascidian diverges from that of the next class. The larva becomes divided into a body and a tail, nearly all the notochord and mesoblast being carried, back into th