392 ZOOLOGY FOR MEDICAL STUDENTS chap. 



under the throat and therefore behind the mouth instead of in front as 

 it is in the ganoids. In the Lung-fish the cement-organ develops out of 

 a local thickening of the ectoderm so that any connexion with the endo- 

 derm — which once existed unless we assume that the cement-organ here 

 is not homologous with that of ganoids — has completely disappeared. 

 Like the external gills the cement-organ is a purely larval structure 

 which disappears later without leaving a trace behind. 



In addition to the typical fish of the groups Elasmobranchii, Teleo- 

 stomi and Dipnoi there must be included in this chapter two groups of 

 creatures, more lowly organized than the true fishes, which are often looked 

 upon as fish in an early stage of evolution but which the present writer 

 regards rather as owing their simpler structure to degeneration and 

 reversion. These groups are the Cephalochorda and the Cyclostomata. 



The group Cephalochorda is formed for the reception of Amphioxus 



M. 



m. 



Fig. 172. 



Amphioxus. an, Anus; at, atriopore ; b.c, fringe of buccal cirri bounding opening of stomo- 

 daeum ; M, myotomes ; m, metapleure ; v, velum. 



— a creature that has excited much interest in zoological speculation 

 through its being regarded as a link between vertebrates and " inverte- 

 brates.'' Although this claim can no longer be allowed Amphioxus 

 remains a creature of much interest from its remarkable mixture of 

 features that are indicative of degeneration or specialization for its 

 peculiar mode of life with others that appear to mark a retention of, or 

 a return to, exceedingly archaic conditions. 



Amphioxus is a small marine creature measuring about 2 inches in 

 length, pointed towards each end, and somewhat flattened from side to 

 side (Fig. 172). It lives embedded in the sand, making an occasional 

 aimless excursion into the superjacent water but soon sinking down 

 again and showing none of the purposeful wandering from place to place 

 directed by definite psychic activity that is seen in an ordinary fish. In 

 correlation with this the highly differentiated head-region with its equip- 

 ment of sense-organs and jaws — so conspicuous a feature of the typical 

 vertebrate — is here entirely absent. 



