72 COMMISSION OF CONSERVATION 



Palps are at first closely allied with the velum, and this is due to the 

 association of the oesophagus and mouth with that organ. The first trace 

 of the organ that can be followed into the palps is found in the larva, in the 

 tissues between the oesophagus and velum. In front of (i. e. above) the 

 oesophagus, and squeezed rather asymmetrically to the right, is a deeply 

 staining, dense band, inclined to be split into two layers by a narrow 

 transverse slit, and also partly constricted into right and left halves 

 (Plate VI, fig. 4, p). It appears to be an invagination in front of, and 

 mostly on the right side of the oesophagus, pointing inwards and upwards 

 towards and ending below the supra-oesophageal ganglion. That it does 

 not belong to the velum is shown by the fact that the yellow, chitinous- 

 looking matter, to be found on the surface and in the creases of the upper 

 and more posterior parts of the velum, dips in between this organ and the 

 velum, thus throwing it rather with the oesophagus, with which it remains 

 when the velum disappears. In my youngest spat (Plate VI, fig. 12) this 

 structure is the same as in the larva, excepting that it is rather larger and 

 closer to the end of the oesophagus (mouth). In those a little older (Figs. 

 21, 23, 24) it doubles across the front of the oesophagus just above the 

 mouth, broadening into a large cavity, that sometimes opens only 

 on the right,side. By downward growth of the anterior wall it is made 

 to receive the mouth, and by growth of the inner and outer edges of the 

 opening (Fig. 25) the lower and upper palps are produced. Spat of a 

 length of 72 already show a single furrow on the inner surface of the an- 

 terior wall and continuing outwards laterally on to the inner surface of the 

 upper palps. With the loss of the velum the oesophagus comes to curve 

 forwards instead of backwards, and there is need of a broader opening to 

 gather in a greater abundance of minute food-organisms, swept forwards 

 or pushed forwards by the gills, which at this period have grown forwards 

 in close proximity to the mouth. In 1 mm. spat (Plate VII, fig. 1) the 

 two edges of the mantle meet above, forming a sort of hood over and in 

 front of the widely open, transversely crescentic mouth (Fig. 2), that 

 opens a little farther back on the right side than on the left (Fig. 3) . The 

 anterior end of the body, between the first gill filaments, where the foot 

 used to be, serves as the posterior (or inferior) support of the mouth, 

 while very short upper and lower palps, possessing only a couple of ridges 

 and furrows, are continuous with the lateral margins of the upper and 

 lower lips and project on each side of the anterior end of the median (left) 

 gill. In 2 mm. spat (Plate V, fig. 34; Plate VII, fig. 13) the palps have 

 increased perceptibly, doubled the number of furrows, turned down, and 

 are seen to be anterior to, but not continuous with, the gills. 



Ryder ('82-84 p. 786)" it would appear probable that both palps and 



gills originated from very nearly the same primitive structure longitudinal 



folds of epiblast that were at first continuous." 



Rice ('83 p. 28) made an observation which (if we imderstand proboscis to mean 

 oesophagus) comes curiously near the truth: "During the first period of attachment 



