CONTENTS. Xlll 



PAGE 



oxiiB 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. — ' I 



Medullary Tube. — Organs of Sense. — Intestine with an Anterior ' 



Eespiratory Portion (Gill-intestine) and a Posterior Digestive \ 



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



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



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 i 



I 



<t 



CHAPTER XIY. | 



GEEM-HISTOEY OF THE AMPHIOXUS AND OF THE | 



ASCIDIAN. i 



I 



Relationship of the Vertebrates and Invertebrates. — Fertilization of the i 



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



a Spherical Germ-membrane 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 j 

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

 roofs) grow out from the Side-wall of the Body, and, by Coales- j 

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

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

 Amphioxus. — The same Gastrula is Developed, which forms \ 

 a Notochord between the Medullary and Intestinal Tubes. — 

 Retrogressive Development of the s'ame. — The Tail with the | 

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

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

 which remains throughout Life in the Stage of the Larval Ascidian 

 and retains the Tail-fin with the Chorda (Chordonia). — General 1 

 Comparison and Significance of the Amphioxus and the Ascidian 439 i 



