THE MOUTH. 81 



growths ; of these, there are four present at the critical stage, 

 the remaining eight being developed later. 



The true mouth of the adult Amphioxus, the development 

 of which has just been described, is the small opening in the 

 velum, or partition separating the buccal cavity from the pharynx 

 (Fig. 11, p. 38). 



The buccal cavity itself is formed by a pair of folds of 

 integument, which appear about the time of formation of the 

 secondary gill-slits. The two folds are at first upper and lower 

 respectively ; the upper fold commencing above the prseoral pit, 

 and becoming continuous posteriorly with the upper margin of 

 the mouth ; while the lower fold arises as a ridge along the lower 

 and hinder border of the mouth, extending in front across the 

 ventral surface to the right side. 



As the mouth assumes its median position the upper and 

 lower folds increase in size, and form the left and right halves 

 of the buccal hood respectively. 



The buccal tentacles appear early, as papilla-like outgrowths 

 from the buccal folds (Fig. 39, OT). They arise at first entirely 

 from the lower, or future right fold, about the time the mouth 

 commences to shift its position, and they do not extend into the 

 left fold until a much later period. The median ventral tentacles 

 are the first to be formed, and the others are added on in suc- 

 cession at either end of the series. Small cartilaginoid skeletal 

 elements are present at the bases of the tentacles from their first 

 appearance, and ultimately give rise to the buccal skeleton. 



6. The Praeoral Pit. 



At the commencement of the larval period, the praeoral pit, 

 which, it will be remembered, is formed from the left anterior 

 gut diverticulum (p. 70), is a small pit with thick ciliated walls, 

 lying on the left side of the anterior part of the head, above and 

 in front of the mouth, and opening to the exterior by a small aper- 

 ture (Fig. 36, DL). When the mouth commences to shift towards 

 the median plane, a ciliated groove is formed, connecting its upper 

 and anterior angle with the aperture of the prseoral pit ; and 

 as the mouth sinks further and further towards the right side 

 the prceoral pit gradually becomes flattened out (Figs. 37, 38, DL),. 



G 



