GIBSON— CBPHALOCHOED A : " AMPHIOXIDES." 2:55 



can be seen of the septum separating rostral cavity and posterior splanchnocoele, so 

 conspicuous in Amphioxides. Probably the obliteration of the former cavity and the 

 formation of " skeletal " tissue from its walls, already seen anteriorly, will ultimately 

 affect its whole extent, since no wide cavity occurs in either cheek of the hood in the 

 adult JBrancliiostoma. 



The ventral rostral canal appears to open into the cavity of the right cheek at its 

 posterior end, just in front of Hatschek's pit. 



An important point of correspondence with A. pelagicus is the identical character of the 

 long canal forming the anterior prolongation of the fin-ray boxes ; it appears immediately 

 above the brain as a small triangular space in the ventral edge of the dorsal skeletal 

 lamella. 



The brain has a well-developed pigment-spot like that of A. pelagicus, but its cavity 

 no longer communicates with the olfactory pit (PL 15. fig. 13). 



The buccal cavity, or pre-oral hood, stiU opens well up on the left-hand side, the 

 downgrowth of the left cheek being not yet nearly completed, although the velar opening 

 (the larval mouth) has already acquired very nearly its definitive adult position. 

 Hatschek's pit, opening into the roof of the hood near its anterior end, has much the 

 same appearance as in the larva ; possibly it lies a little more towards the right. The 

 tall ciliated epithelium of its right wall is continued into a ciliated tract, which passes 

 downwards and backwards in the right wall of the hood to end in front of the velum. 

 This is evidently the representative of the ventral part of the larval pre-oral organ, 

 destined to become the " Raderorgan " of the adult ; how far it has taken on the 

 characters of the latter the preservation is not good enough to show. 



The buccal skeleton, mouth, and stomoccele {oide figs. 15-17) show in the 

 main the characters described by Van Wijhe (1902) for the adult Branchiostoma. 

 Such differences as occur are probably, most if not all of them, the marks of youth. 

 The inpushing of the buccal skeleton into the cavity of the hood, so marked a feature in 

 the adult, is here only just beginning; consequently the region termed "Lippenstiel" 

 by Van Wijhe does not yet exist as such, and the two cavities which occupy it — right 

 and left "cava epipterygia" — are still directly continuous with the dorsal parts of the 

 stomoccele ; the same is true of the greater part of the outer lip-cavity. But as regards 

 the separation of the "cava" from one another and from the epibranchial ccelom, and 

 their communication with the pterygocceles and — via the ccelomic canals of the first pair 

 of gill-bars — with the hypobranchial coelom, the adult conditions are already attained. 



Perhaps the most notable feature of the stomoccele is its great extension dorsally on 

 either side, seen in figs. 15 and 16. Its dorsal portions are entirely obliterated in the 

 adult Branchiostoma, except for a small remnant on the right, by the enlargement of 

 the velum and its cavity. Here, on the other hand, there is a spacious dorsal cavity 

 on both sides, that of the right directly continuous with the rostral cavity, that of the left 

 prolonged a considerable distance forward as a small ccelomic space lying external to 

 Hatschek's nephridium (fig. 15). 



Taking origin below the aorta, and inserted into the outer wall of the left division of 

 the stomoccele, is a long muscle (seen in fig, 16), which is evidently a remnant — probably 



