Metazoa Rana. 263 



It will have been seen, so far, that the great point of 

 advance in organisation which Amphioxus shows over Lum- 

 bricus is the possession by the former of an endoskeltton 

 supporting a dorsally placed nervous system. Further, as a 

 consequence of the mode of formation of the nervous 

 system, a transverse section of the body of Amphioxus 

 exhibits two tubes a large ventral y placed ccelom, and a 

 small dorsally placed nervous canal. Lumbriats, on the 

 other hand, in transverse section, exhibits only one tube 

 the alimentary canal the nervous system lying free in the 

 coelom. 



Finally, although the alimentary system of the worm is 

 scarcely less complicated than that of Amphioxus, yet the 

 respiratory system in the latter is greatly in advance of the 

 simple dermal respiratory mechanism in Lumbricus. 



We have now completed our study of this very simple 

 chordate, and we have now to consider a more highly de- 

 veloped type of the class, so as to obtain some idea as to 

 how the simple organs of Amphioxus become modified in 

 these higher forms. 



SECTION II. AMPHIBIA RANA. 



In the preceding section we noted that the great point 

 of advance in the structure of the higher animals was the 

 possession of a backbone, or vertebral column, above which 

 (i.e. dorsally) lay the nervous system in the form of a tube 

 developed from the outer layer of the body of the embryo, 

 and beneath which (i.e. ventrally) was situated the body- 

 cavity, a space containing the chief organs of alimentation, 

 circulation, purification, and reproduction. In Amphioxus 

 the vertebral column we found to be of very simple structure, 

 being merely a rod of cellular tissue, or notochord, but en- 

 closed by a tough membrane continuous with the septa 

 separating the myotomes from each other, and present also 

 as a sheath round the dorsally placed nerve-cord. In forms 



*S 4 



