'472 



SUB-PHYLUM CEPHALOCHORDA. 



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 

 into 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 seg- 

 mentation, of nephridia, and any trace of ccelom 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 by 

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

 gonads and asymmetrical metapleura, should be recognised. 



B 



FlG. 254A. Small portions of excretory organs of Amphioxus (A), 



and the Polychaete, Phyllodoce (B). After Goodrich. 

 6\, Solenocyte; N.^ nucleus; .#., flagellum ; 7 1 ., tube; C., excretory canal. 



