CHAPTER XIII. 



THE STRUCTURE OF THE BODY OF THE AMPHIOXUS 



AND OF THE ASCIDIAN. 



Causal Significance of the Fundamental Law of Biogeny. — Influence of 

 Shortened and Vitiated Heredity. — Kenogenetic Modification of Palin- 

 genesis. — The Method of Phylogeny based on the Method of Geology. — 

 Hypothetic Completion of the Connected Evolutionary Series by Appo- 

 sition of the Actual Fragments. — Phylogenetic Hypotheses are Reliable 

 and Justified. — Importance of the Amphioxus and the Ascidian. — 

 Natural History and Anatomy of the Amphioxus. — External Structure 

 of the Body. — Skin-covering. — Outer-skin (^Epidermis^ and Leather-skiu 

 (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). — Move- 

 ment of the Blood. — Lymph-vessels. — Ventral Canals and Side Canal? 

 — Body-cavity and Gill-cavity. — Gill-covering. — Kidneys. — Sexual 

 Organs. — Testes and Ovaries. — Vertebrate Nature of Amphioxus. — Com- 

 parison of Amphioxus and Young Lamprey (Petromyzon). — Comparison 

 of Amphioxus and Ascidian. — Cellulose Tunic. — Gill-sac. — Intestine. 

 — Nerve-centres. — Heart. — Sexual Organs. 



** The primitive history of the species is all the more fully retained in 

 its germ-history in proportion as the series of embryonic forms traversed is 

 longer ; and it is more accurately retained the less the mode of life of the 

 recent forms differs from that of the earlier, and the less the peculiarities 

 of the several embryonic states must oe regarded as transferred from 9, later 

 to an earlier period of life, or as acquired independently." — Fkitz Mulleb 

 (1864). 



In turning from the germ-history to the tribal history 

 of Man, we must constantly bear in mind the causal connec- 



