92 Observations upon the Habits, Structure and [| February, 
edge has become lip-like and three or four finger-like prolonga- 
tions have formed from the exoderm of the lip. 
At the same time with the forward growth of the left side 
wall, the tentacles of the left side have grown longer, increased in 
number by the addition of one or two prolongations from that 
portion of the wall of the cavity anterior to those tentacles al- 
ready formed, and become strengthened by cartilaginous supports 
formed .in the tissue of each tentacle, as tentacular segments, al- 
ready noticed in the adult, of the left branch of a ring which de- 
velops around the edge of the mouth aperture from the base of 
the cartilaginous ring of the pharynx. 
At this period the mouth tentacles have very little shave in 
guarding the mouth cavity from the entrance of large or injurious 
particles, this duty being delegated almost entirely to the tenta- 
cles of the pharyngeal ring, which were formed very soon after 
the opening of the passage-way through the bottom of the cilia- 
ted pit into the pharynx. And while these pharyngeal tentacles 
are thus of much greater importance to the young than to the 
adult animal, they are always of service, as has been noticed upon 
a previous page, in ejecting large bodies which may have suc- 
ceeded in passing the network formed by the mouth circlet and 
getting into the mouth cavity. The further development of the 
mouth cavity is comparatively simple; the left side continues its 
downward and forward growth, until it is upon a level with the 
right edge, and the mouth opening becomes a median, ventral 
aperture just posterior to, and guarded by the rather blunt pro- 
boscis of the animal; the right branch of the mouth ring with 7s 
segments, forms in and along the edge of the right wall, and the car- 
tilaginous supports grow out into the right tentacles; the remain- 
ing tentacles, to the number of ten or eleven on either side, mak- 
ing about thirty-one in all, gradually form along the sides, arch- 
ing inward and forward, so that those of one side overlap and 
interlace with those of the other, and the mouth cavity assumes 
its normal adult appearance (Fig. 1, Pl. 1). 
By following carefully the development of the mouth cavity of 
Amphioxus, as I have sketched it in the previous pages, it will 
be seen to be a true, although somewhat irregular, introversion : 
or inward growth of the exodermic tissue to meet the endoderm _ : 
of the central canal, and hence is homologous in character with 
the mouth cavity of the higher vertebrates. It has heretofore : 
