AMPHIOXUS LANCEOLATUS. 
369 
bounded by the oral hood “ mouth,” since the true mouth 
above indicated exists before the oral hood is formed. The 
oral hood is the preeoral portion of the epipleural folds, which 
post-orally give rise to the “atrial/’ “branchial/’ or “epi- 
pleural” chamber. The true mouth is that which has been 
compared by Huxley (9), whose nomenclature is followed by 
Langerhans, to the velum palati of Cyclostome fishes. It 
has twelve delicate tentacles projecting freely from its margin 
backwards into the pharynx. The grouping of these has 
not hitherto been satisfactorily figured in any account of 
Amphioxus; they are represented in PI. XXXVI B, fig. 12. 
They were seen by Rathke (8) and by Joh. Muller (1), who 
figure them as seen when the sphincter is slit open ; their 
minute structure is figured and described by Langerhans, who 
calls them papillae. 
It is difficult to assign a position in relation to the myo- 
tomes to those organs which lie more ventrally than the 
segmented musculature of the body wall. The myotomes are 
separated from one another by connective-tissue septa, each of 
which, instead of being vertical, is directed obliquely upwards 
and backwards in the dorsal half, and obliquely downwards 
and backwards in the ventral half, of its extent ; and as the 
myotome becomes very narrow and almost horizontal before it 
disappears ventrally, it is not possible to assert with any 
assurance that structures lying below the region into which 
the myotomes extend are behind or in front of any given one 
of these obliquely directed structures. 
I am inclined to the view that the oral sphincter is morpho- 
logically in front of the first myotome, though its position 
coincides approximately with a vertical line drawn through the 
anterior angle of the seventh. This back-pushed position of 
the ventrally placed organs in relation to the myotomes of the 
body wall is characteristic of Amphioxus, and is connected 
with the establishment of an independent metamerism of the 
alimentary canal, which, after the early larval condition, seems 
to be in no definite relation to the metamerism of the body wall. 
The atriopore or ventral median aperture of the peri- 
