CONTENTS. Xlii 



PACK 



oxns 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. Organs 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). 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 Amphioxns. 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. 



GERM-HISTORY OF THE AMPHIOXUS AND OF THE 

 ASCIDIAN. 



Relationship of the Vertebrates and Invertebrates. Fertilization of the 

 Amphioxns. 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 

 Gastrnla 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) grow 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 Gastrnla 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 throughout Life in the Stage of the Larval Ascidian 

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

 Comparison and Significance of the Amphioxus and the Ascidian 439 



