560 CEPHALOCHORDA. 



velar tentacles soon appear. There are at first four of these, two 

 lateral, one upper and one lower; later, the number increases to 

 twelve. In the lower of the two secondary oral folds, the first rudi- 

 ment of the cartilaginous skeleton that supports the oral cirri (Fig. 

 296, be) soon appears in the form of rounded thickenings of the 

 mesodermal tissue. Each of these cartilaginous spherules corresponds 

 in later stages to an outgrowth of the edge of the mouth which gives 

 rise to a cirrus. New cirri form in the lower lip in front of and 

 behind those already formed, while, from the condition of the adult, 

 BAY LANKESTEK concluded that the median cirri of the ventral edge 

 of the mouth were the last to arise. 



v* p 



FIG. 296. The same stage as in Fig. 292 B seen from the left side (after WILLEY). 1, 

 2, 14, first, second, and fourteenth primary gill-clefts, be, buccal cirri ; fl, peri- 

 pharyngeal ciliated band ; fh, dorsal fin-cavities ; mr, oral fold ; np, neuropore ; 

 nph, nephridium of HATSCHEK ; p, atrial cavity ; v, velum ; w, wheel-organ (part 

 of pre-oral pit). 



After the oral hood has formed, those organs which opened in the 

 immediate proximity of the larval mouth have to open into the 

 secondary oral cavity. These are the club-shaped gland and the 

 pre-oral pit which marks the aperture of the sensory organ derived 

 from the left anterior entoderm-vesicle. 



The lower of the two folds which form the definitive oral aperture is 

 continued forward without break into the unpaired fin (BAY LANKESTER). 

 RAY LANKESTEB interprets the oral folds as the anterior continuation of the 

 so-called epipleura (lateral walls of the atrial cavity) and HATSCHEK (No. 8) 

 has also adopted this view. According to WILLEY, the right half of the oral 

 hood arises essentially in continuity with the right metapleur. The left half, 

 however, is entirely independent of the left metapleur. The latter condition, 

 WILLEY thinks, may possibly be secondary. 



Another view has recently been adopted by VAN WUHE (No. 22), who 

 maintains that both the halves of the oral hood belong exclusively to the 

 left side of the body. This conclusion was arrived at from a consideration of 

 their innervation as well as from the fact that only the cavity of the left 

 lateral fold is continued into the lip on the outer side of the massive external 

 lip-muscle. The definitive mouth of Amphioxus is thus, according to YAH 



