INTROD UCTION. 



the liver ducts, the stomach thus being entirely absent. The 

 notochord extends along the whole length of the animal. These 

 forms, however, differ from the vertebrates in the absence of 

 vertebrae, in the peculiarities of the central nervous, system 



Fig. 3. Diagram of Amphioxus, chiefly after Boveri. A, atrium ; AP, atrio- 

 pore ; B, branchial clefts ; G, gonads ; Z, liver ; M, mouth surrounded by cirri ; 

 MY, myotomes ; N, nephridia ; NC, notochord ; S, spinal cord ; V, vent. 



and the nerves which arise from it, in the total absence of a 

 heart, of paired eyes, etc., as well as in the relations of excre- 

 tory organs, etc. The species are few in number, and are all 

 marine, being found in the warmer seas of all parts of the 

 world ; on our coasts as far north as the mouth of the Chesa- 

 peake. 



The Enteropneusti are decidedly worm-like in appearance, 

 and their pertinence to the chordate phylum is denied by 

 many. The so-called notochord is but a small diverticulum 

 from the alimentary tract without skeletal character, while it is 

 not found that the segmentation of the body is the same as that 

 in the other chordates. The best-known form — Balanoglossus 

 — was long considered a worm. It lives in the sand of the 

 seashore in many parts of the world. Other allies are so dif- 

 ferent in appearance (^Rhabdopleura, Cephalodiscus') that they 

 were long regarded as Polyzoa. 



For further details concerning these forms reference must 

 be made to works upon invertebrates, and to the special papers 

 dealing with them. With this brief reference they must be 

 dismissed here ; for the purpose of the present work is to deal 

 with the single subphylum, Vertebrata. 



In the second or systematic portion of this volume, the dif- 

 ferent subdivisions of the group of vertebrates will be defined ; 



