528 CEPHALOCHORDA 



line. Many other intricate changes occur, and the young 

 Amphioxus begins to burrow in the sand. By the process 

 known as " symmetrisation " of the larva, the apparent 

 symmetry of the adult is produced. The adult position 

 of the anus and of the olfactory pit, both to the left side, 

 and the position of the unpaired liver diverticulum, show 

 how partial this process is. 



Relations of Amphioxus and Tunicates 



The above account of Amphioxus will in its details 

 recall to the student the description of Tunicates. It is 

 indeed remarkable that the resemblance should be so much 

 stronger in minor anatomical points than in broad outline, 

 but this is in part explained by the very marked degenera- 

 tion displayed by the adult Ascidians. 



The following important resemblances should be noticed : 

 — In both cases the walls of the pharynx are perforated by 

 numerous slits, which open, not directly to the exterior, but 

 mto an atrial or peribranchial chamber, formed from the 

 ectoderm, and with a single external aperture. In both, the 

 pharynx has a distinct ventral glandular endostyle, and a 

 dorsal fold (Tunicates) or groove {Amphioxus), connected 

 anteriorly to the endostyle by means of a ciliated band. 



On the other hand, the Ascidians differ from the lancelets 

 in many ways, e.g. the sessile habit, the presence of the test, 

 of a heart, and of genital ducts ; the absence of segmenta- 

 tion, of nephridia, and any trace of coelom in the adult ; 

 the U-shaped alimentary canal ; the power of budding, so 

 common in sedentary animals ; and the hermaphroditism. 



The detailed study of development yields similar series 

 of facts — marked resemblances coupled with marked 

 differences ; among the latter, the absence in Ascidians of 

 the segmented ccelomic pouches of lancelets is especially 

 noteworthy. It is probable that Lancelets and Tunicates 

 are descended from a common primitive chordate ancestry. 



In strict usage the name Amphioxus should be replaced bv 

 Branchiostoma, and another genus, Asymmetron, with uniserial (right) 

 gonads and asymmetrical metapleura, should be recognised. 



