38 ZOOLOGY SECT. 



typical condition, at least in the adult. In Balanoglossus, Cephalo- 

 discus, and Rhabdopleura. the " notochord " is rudimentary, and in 

 nearly all Tunicata it is present only in the embryo. In 

 Rhabdopleura the gill-slits are absent, and in that genus as well 

 as in Cephalodiscus and the adult Tunicata the nervous system 

 is represented by a single solid nerve-centre or ganglion, the 

 neuroccele being absent. In Balanoglossus, moreover, there is- 

 a ventral as well as a dorsal nerve-cord, and it is only in the 

 anterior portion of the latter that the neurocoele is represented. 



In the Vertebrata, 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-Avail is segmented, and the renal organs arise as 

 meso-nephridia. 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 intestine circu- 

 lates through it before passing into the general current, thus 

 giving rise to what is called the hepatic portal system of blood 

 vessels. 



There are two primary sub-divisions of Vertebrata of very 

 unequal size. 



DIVISION A. ACRANIA. 

 Including only the little fish-like Lancelets. 



DIVISION B. CRANIATA. 

 Including Fishes, Amphibians, Reptiles, Birds, and Mammals. 



DIVISION A. ACRANIA. 



The division Acrania contains a single family, the Bro.nchios- 

 tomidce, containing two genera. Brancliiostoma (usually known by 

 the name of one of its sub-genera, Ampliioxus), and Asi/nunetron. 

 The differences between the genera and species are comparatively 

 insignificant, and the following description will deal exclusively 

 with the best known and most thoroughly investigated species, 

 the Lancelet or Amphioxus, Amphioxus lanceolatus, found in the 

 English Channel, the North Sea, and the Mediterranean. 



