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. 
Fic. 25§4A.—Small portions of excretory organs of Amphzoxus (A), 
and the Polychete, Phyl/odoce (B).—After Goodrich. 
S., Solenocyte; ¥., nucleus; /*Z.., flagellum; 7., tube; ZC., excretory canal. 
