222 PERCY SLADEN TETJST EXPEDITION. 



The opening of the splancbnocoele into the cavity of the first right myotome is always 

 wide and conspicuous. 



The Pre-oral Organ. 



The gross structure of this organ as seen in my specimens does not quite tally with 

 Goldschmidt's description. He describes its opening as a V-shaped slit, with its apex 

 posterior, and its obliquely-directed lower limb the shorter (forming a direct opening into 

 the ventral diverticulum of the organ). In my specimens, on the other hand, the 

 opening always forms a simple horizontal slit : its lower lip may be slightly depressed 

 posteriorly, so that here the ventral diverticulum may be said to open directly to the 

 exterior : generally speaking, however, this ventral cavity opens into Goldschmidt's 

 " Havipthohle " throughout its extent, and is continued into no blind process anteriorly, 

 though there may be one posteriorly, as seen in fig. 7. 



The lips of the opening are continued directly into those of the ciliated groove leading 

 to the moutb: the upper forms a prominent ridge overhanging the groove, at the bottom 

 of which the mouth-opening breaks through ; the lower is rounded ofi" and passes into 

 the ventral wall of the stomodseum. 



In a few of the oldest specimens of A. pelagicus the pre-oral organ was particularly 

 widely open and a conspicuous bunch of stiif, hair-like cilia was seen immediately 

 behind it, springing apparently from the lower margin of the groove. A similar bunch 

 of cilia occurs in the same place in very young Amphioxns larvae (as figured by Hatschek, 

 1 8 8 1 ) and also — though less conspicuous — in larvae about to metamorphose ( Willey. 1 89 1 ), 

 but was not seen in any except a few A. pelagicus, which further differed from the rest 

 in the condition of their gill-slits : the head of one of them is shown in fig. 4. 



The epithelium of the pre-oral organ appears to be everywhere ciliated, and the 

 glandular differentiation of Hatschek's pit, described by Andrews and Van Wijhe (1902), 

 is not yet visible. 



The Mouth and adjoining Organs. 



These are difficult to make out satisfactorily in whole preparations, owing to the over- 

 hang of the upper lip of the stomodseum, which conceals the lower lip in all except well- 

 expanded specimens. Pig. 3 gives an idea of its normal relations in A. pelagicus. 



Goldschmidt describes a single " Mundschliesser " muscle having its origin anteriorly, 

 and its insertion posteriorly, below the chorda. It appears to me truer to say that there 

 are distinct anterior and posterior muscles, as in the Branchiostoma larva (Van Wijhe, 

 1906), each taking origin below the chorda, and inserted into the somatopleure of the 

 lower lip. Their insertions overlap in the middle region of the lip, but the fibres seen 

 here are few compared to those seen further forward and further back. 



The lower limb of the endostyle may extend to the extreme end of the ventral 

 caecum of the pre-oral gut, or may not appear anterior to the mouth-region. In either 

 case, its anterior end lies to the left of the topographical, as of the morphological, middle 

 line : followed backwards in sections, it is soon seen to cross over onto the right, while on 

 the left there passes back from its most anterior portion the left peripharyngeal band. 



