232 PROFESSOR MARSHALL AND EDWARD J. BLES. 



In the tail the division of the muscles into segments is very 

 apparent, the myotomes, as in the earlier stages, being > shaped, 

 with the angle directed forwards, and the dorsal limb of the > 

 shorter than the ventral, as in the adult Amphioxus. 



2, Internal Anatomy. 



The brain, which has advanced considerably in development, the 

 cerebral hemispheres being now well formed, stops a considerable 

 distance from the anterior end of the head, this change being due 

 to the large growth forwards of the labial region (Fig. 13). 



The mouth leads into a buccal cavity, which is rather small, and 

 squarish in transverse section. Both its roof and floor bear prominent 

 papilla?, but there is no trace of the tongue. The posterior nares 

 now open into the roof of the buccal cavity by apertures guarded by 

 valve-like folds of epithelium. 



The pharynx, with which the buccal cavity is continuous posteriorly, 

 is shallow dorso-ventrally, but very wide from side to side. In 

 horizontal section it is triangular, with the apex in front; the posterior 

 wall being almost straight, and perforated dorsally in the median 

 plane by the very narrow oesophagus. 



The gill clefts are slit-like, and perforate the sides and ventral wall 

 of the pharynx. As seen from the ventral surface, the slits run 

 outwards and backwards obliquely across the floor of the pharynx, 

 so that a single transverse section may cut all four branchial arches 

 and clefts of both sides, the anterior arches appearing at the outer 

 sides of the section, and the posterior arches next to the median 

 plane, (cf. Fig. 15, in which these relations are shown at a rather 

 later stage). 



With regard to the individual clefts, the hyomandibular cleft does 

 not open to the exterior at this, or, so far as our observations go, 

 at any other period. The first branchial cleft opens, but by a very 

 small aperture, into the anterior end of the opercular cavity. The 

 second, third, and fourth branchial clefts open widely into the 

 opercular cavity. 



The operculum, or opercular membrane, arises, as already noticed 

 (Fig. 12, O F), as a backwardly projecting fold from the outer and 

 posterior margin of the hyoid arch. The folds commence at the 

 sides, but very early extend down to the mid-ventral line, so as to 



