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. — Kenogeuetic Modification of Palin- 

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

 Hypothetic Completion of the Connected Evolutionary Series by Appo- 

 sition of the Actual Fragments. — Phylogenetic Hypotheses are Eeliable 

 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-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). — Move- 

 ment 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. — 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 difi'ers from that of the earlier, and the less the peculiai-ities 

 of the several embryonic states must be regarded as transferred from a later 

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

 (1864). 



In turning from the germ-history to the tribal history 

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



