LATER LARVAL STAGES. 559 



The metapleural folds have frequently been homologised with the 

 primary paired lateral tins of the Vertebrata (BAY LANKESTER and 

 WILLEY, HATSCHEK). According to HATSCHEK (No. 8), they are 

 merely special parts of a system of ventral folds which in the most 

 anterior part of the body develops as the unpaired ventral fin of the 

 rostrum, in the oral region forms the lateral buccal wall, in the 

 branchial region the metapleural fold, and, finally, behind the atrio- 

 pore, the unpaired ventral fin which extends in front of and behind 

 the anus. The " cavities of the lower folds " which develop in the 

 sub-atrial ridges are said to be the cavities of the unpaired ventral 

 fin. 



\Yo have already alluded (p. 550) to a ciliated band lying on the 

 right side of the pharyngeal wall in front of the club-shaped gland 

 (Fig. 289, fi). This ciliated band, in which a somewhat clearer inner 

 zone can be distinguished, resembles in shape the club-shaped gland, 

 so that, in the illustration, it looks like a shadow or a reduplication 

 of the latter. It is the rudiment of the endostyle and, even in early 

 stages, appears bent on itself (Fig. 292 A, e*) t being divided into a 

 shorter upper and a longer lower half. The endostyle-rudiment then 

 proceeds backward (Fig. 292 B) passing over the club-shaped gland 

 into the space between the primary and secondary clefts. It thus 

 comes to overlie the branchial artery. During the shifting mentioned 

 above as taking place in the branchial region, the endostyle-rudiment 

 passes from the right side of the body to th.e ventral median line 

 (Fig. 293, fii). The upper half of the bent rudiment becomes the 

 right and the lower half the left part of the endostyle of the adult. 

 At an early stage, two ciliated arches, the peripharyngeal bands, are 

 . to be seen ascending from the anterior end of the endostyle-rudiment 

 to the dorsal side of the pharyngeal wall (Fig. 296, /), and then 

 continue backward on either side of the dorsal middle line as the 

 ciliated hyperpharyngeal (epipharyngeal) bands. The proximal part 

 of them corresponds to the ciliated arches (pericoronal arches or 

 peripharyngeal bands) of the Ascidian ( WILLEY). 



The formation of the definite oral aperture has been described in 

 detail by WILLEY. The oral aperture shifts from the left side of 

 the body forward and ventral wards, so that it finally occupies a 

 median symmetrical position on the ventral side. At the same time, 

 it becomes grown over by a secondary fold of the body- wall (Fig. 296, 

 >/?/), the stomodaeum or oral hood being thus formed. The primary 

 oral aperture of the larva shifts to the back of the buccal cavity and 

 its lips are retained as the velum (v) ; here the first rudiments of the 



