24 ANATOMY OF AMPHIOXUS. 



as the atriopore. Although the details in the formation of 

 the atrium are not exactly such as they were supposed to 

 be by Rolph (see below), yet the end-result is virtually the 

 same, and his work marks a distinct advance in our knowl- 

 edge of the structure of Amphioxus, by showing that the 

 epithelium lining the walls of the atrial chamber is not 

 peritoneal, but is derived by a process of in-folding, from 

 the ectodermic covering of the surface of the body. In 

 other words, the atrial cavity, like the opercular cavity of 

 the Amphibian tadpole, is lined by ectoderm. 



Viscera. 



A bird's-eye view of the internal organs, as exposed by 

 cutting the animal open ventrally by incisions extending 

 forwards and backwards from the atriopore, is shown in 

 Fig. 9. First and foremost, our attention is arrested by 

 the relatively enormous pharynx occupying more than half 

 the length of the body, with its right and left perforated 

 walls and parallel gill-bars abutting at the mid-ventral line 

 on the endostyle. 



The alimentary canal is seen in the dissection to have a 

 perfectly straight course between mouth and anus, with 

 no windings whatever. Growing out ventrally from what 

 may be termed the pyloric region of the intestine, a short 

 distance behind the pharynx and in front of the atriopore, 

 there is a large diverticulum ending blindly in front, which 

 in the adult lies for the greater part of its extent applied 

 against the right wall of the pharynx (Fig. 9). This is 

 the so-called hepatic cactmi, corresponding to the liver of 

 higher forms. The permanent condition of the liver in 

 Amphioxus is comparable to its embryonic condition in the 

 Vertebrates, where it attains a much more complicated 

 structure in the older stages by subsequent branching and 



