22 ANATOMY OF AMPHIOXUS. 



INTERNAL ANATOMY. 

 At rial Cavity. 



In making a dissection of a frog or a fish, as soon as the 

 body-wall is cut through, we find ourselves groping about 

 in a large cavity in which the viscera lie. This is the 

 body-cavity or peritoneal cavity, or, again, the ccelom. 



If we slit open the ventral body-wall of Amphioxus, we 

 discover what appears to be an exactly similar cavity. It 

 is, however, not the ccelomic cavity, but the peribranchial 

 or atrial cavity, into which the pharyngeal gill-slits open. 

 The older anatomists, including Johannes Miiller, regarded 

 it as the true body-cavity, and the latter author was forced 

 to the conclusion that Amphioxus differed fundamentally 

 from all the other Vertebrates in that the gill-slits opened 

 into the peritoneal cavity. Although that condition of 

 things was hard to imagine, yet it seemed to be obviously 

 the case, since the reproductive organs appeared to lie in 

 the same cavity, and it went without saying that a cavity 

 containing the gonads could only be the peritoneal cavity. 

 In reality, the gonads do not lie in this cavity ; they only 

 project into it and lie in a space of their own which is 

 separated from the atrial cavity by a double-layered mem- 

 brane. (Cf. Fig. 2.) 



HUXLEY threw some light on the matter in 1874, when 

 he compared the atrial or peripharyngeal cavity of Amphi- 

 oxus to the opercular cavity which surrounds the gills of 

 the tadpoles of the frog and tailless Amphibia generally. 

 In the case of the tadpole, as is well known, there are some 

 four pairs of gill-slits which open at first directly to the 

 exterior. Subsequently an opercular fold grows backwards 

 over them as in fishes, but with this difference, that in the 



