chap, in.] CHORDATA. 87 



there would seem to be degradation ; these are the 

 Cephalocliordata ; and, lastly, we have the true 

 Vertebrata or Craniata. 



A. Cephalochordata. Of these the only exam- 

 ple is the Lancelet or Amphioxus, in which the noto- 

 chord, pointed at either extremity, extends from one 

 end of the body to the other ; the number of gill slits 

 is very great, and they are covered over by an out- 

 growth of the body wall which grows down on either 

 side, and unites along the ventral line, leaving a pore 

 for the exit of the water (atrial pore). The original 

 segmentation of the muscles of the body is not ob- 

 scured ; the mouth is over-hung by a projecting hood, 

 and furnished with a number of tentacles (cirri) ; the 

 liver is represented by a very slight, blindly ending 

 outgrowth of the enteric tube, and renal organs are very 

 obscurely indicated ; there is no centralised heart, and 

 appendages are completely wanting. The eye is only a 

 pigment spot, and no signs of an ear have been detected. 

 B. Urocliordata. In no division of the animal 

 kingdom has the value of the study of development 

 been of more importance than in this, for it has 

 revealed the presence of a iiotocliord, and the essen- 

 tial resemblance between their gill clefts and those of 

 the Cephalocliordata ; while in none has the applica- 

 tion of the principle of degeneration (Dohrn ; Lan- 

 kester) been more instructive. 



In but few forms is the notochord retained 

 throughout life, and in these it is found in the tail 



O 



only, Perennicliordata (e.g. Appendicularia) ; in 

 the rest, Caducicnordata, the caudal notochord is 

 present in the larva only, or is never developed at all ; 

 in these, just as in Amphioxus, outgrowths of the body 

 wall enclose the true sides of the body, and give rise 

 to an atrial chamber, by whose pore the water of re- 

 spiration, and often also the waste matters of digestion 

 finally make their way to the exterior (Fig. 44r). 



