44 ZOOLOGY sect. 



In the Euchorda, on the other hand, what have been called 

 the three fundamental chordate peculiarities are fully and clearly 

 developed. There is always a distinct notochord extending as a 

 longitudinal axis throughout the greater part of the elongated 

 body, and either persisting throughout life, or giving place to an 

 articulated vertebral column or backbone. The central nervous 

 system remains throughout life in the form of a dorsal nerve- 

 tube or neuron containing a longitudinal canal or neurocoele, 

 and the pharynx is always perforated, either throughout life or 

 in the embryonic condition, by paired branchial apertures or gill- 

 slits. In addition to these characters, the mouth is ventral and 

 anterior, the anus ventral and posterior ; the muscular layer of 

 the body-wall is primarily segmented, and the renal organs arise as 

 a series of paired tubules which may represent either nephridia 

 or coelomoducts. Moreover, there is always an important 

 digestive gland, the liver, developed as a hollow outpushing of 

 the gut, and distinguished by the fact that the blood from the 

 alimentary canal circulates through it before passing into the 

 general current, thus giving rise to what is called the hepatic 

 portal system of blood-vessels. 



The sub-phylum Euchorda comprises two sections of very un- 

 equal extent. 



Section I. — Acrania (Cephalochorda). 

 Including only the little fish-like Lancelets. 



Sectiox II. — Craniata (Vertebrata). 



Including Cyclostomes, Fishes, Amphibians, Reptiles, Birds, and 

 Mammals. 



SECTION I.-ACRANIA (CEPHALOCHORDA). 



The section Acrania includes only two families, the Branchio- 

 stomida: — containing the genera Branchiostoma (usually know^n by 

 the name of one of its sub-genera, Amiohiooms), Bpiyonichthys, and 

 perhaps some others; and the Amphioxididm — containing the 

 pelagic genus Amphi'oxides, which, however, may not be an adult 

 form. The differences between the genera and species of 

 Branchiostomida? are comparatively msignificant, and the follow- 

 ing description will deal exclusively with the best known and 

 most thoroughly investigated species, the Lancelet, AmjMoaus 

 lancrolatus, found in the English Channel, the North Sea, and 

 the ]\redi terra nean. 



Amphioxus is a small transparent animal, occurring near the 

 shore and burrowing in sand ; its length does not exceed 5-8 cm., 



