CONTENTS. Xlll 



PAGE 



oxus and the Ascidian. — Natural History and Anatomy of the 

 Amphioxus.— External Structure of the Body. — Skin-covering. — 

 Outer-skin (^Epidermis) and Leather-skin (Corium). — Notochord. — 

 Medullary Tube. — Oi'gans of Sense. — Intestine with an Anterior 

 Respiratory Portion (Gill-intestine) and a Posterior Digestive 

 Portion (Stomach-intestine). — Liver. — Pulsating Blood-vessels. — 

 Dorsal Vessel over the Intestine (Gill-vein and Aorta). — Yentral 

 Vessel under the Intestine (Intestinal Vein and Gill-artery). — 

 Movement of the Blood. — Lymph-vessels. — Ventral Canals and 

 Side Canals — Body-cavity and Gill-cavity. — Gill-covering. — 

 Kidneys. — Sexual Organs. — Testes and Ovaries. — Vertebrate 

 Nature of Amphioxus. — Comparison of Amphioxus and Young 

 Lamprey (Petromyzon). — Comparison of Amphioxus and Ascidian. 

 — Cellulose Tunic. — Gill-sac. — Intestine. — Nerve-centres. — Heart. 

 — Sexual Organs ... ... ... ... ... ... 406 



CHAPTER XIV. 



GEEM-HISTOEY OF THE AMPHIOXUS AND OF THE 



ASCIDIAN. 



Kelationship of the Vertebrates and Invertebrates. — Fertilization of the 

 Amphioxus. — The Egg undergoes Total Cleavage, and changes into 

 a Spherical Germ-membi^ne Vesicle (Blastula). — From this the 

 Intestinal Larva, or Gastrula, originates by Inversion. — The 

 Gastrula of the Amphioxus forms a Medullary Tube from a Dorsal 

 Furrow, and between this and the Intestinal Tube, a Notochord: 

 on both Sides the latter is a Series of Muscle-plates ; the Matemera. 

 — Fate of the Four Secondary Germ-layers.— The Intestinal Canal 

 divides into an Anterior Gill-intestine, and a Posterior Stomach- 

 intestine. — Blood-vessels and an Intestinal-muscle Wall originate 

 from the Intestinal-fibrous Layer. — A Pair of Skin-folds (Gill- 

 roofs) gi-ow out from the Side-wall of the Body, and, by Coales- 

 cence, form the Ventral Side of the Large Gill-cavity. — The 

 Ontogeny of the Ascidian is, at first, identical with that of the 

 Amphioxus. — The same Gastrula is Developed, which forms 

 a Notochord between the Medullary and Intestinal Tubes. — 

 Retrogressive Development of the same. — The Tail with the 

 Notochord is shed. — The Ascidian attaches itself firmly, and 

 envelops itself in its Cellulose Tunic. — Appendicularia, a Tunicate 

 which remains tlu'oughont Life in the Stage of the Larval Ascidian 

 and retains the Tail-fin with the Chorda (Chordonia).— General 

 Comparison and Significance of the Amj)hioxus and the Ascidian 439 



