DEVELOPMENT OF BALANOGLOSSUS KOWALEVSKII. 113 
(figs. 57 and 56). Immediately behind this downward exten- 
sion lies the anterior end of the united skeletal rods, which 
here attains its greatest thickness, almost filling the sheath of 
the notochord, the tissue of which is here almost suppressed. 
In old specimens the shape of the anterior parts of the 
notochord becomes rather irregular in section. 
In that part of its course which lies behind the proboscis the 
notochord in the adult is more or less elliptical in section, 
containing a large and somewhat irregular lumen. Its tissue 
is here greatly reduced, and this reduction appears to progress 
regularly as the animal grows older. In fig. 60 the appearance 
of the notochord in such an old adult is shown. Degeneration 
has progressed far, leaving the notochord as a space surrounded 
by vacuolated cells enclosed in asheath. With this sheath are 
connected the skeletal rods, which attain a great size. Cen- 
trally, on the dorsal side, between the notochord and the gut, 
lies the principal rod ; this is formed by the uniting of the two 
rods (figs. 37, 38, &c.), whose development has already been 
described. This fused portion is now diamond-shaped in sec- 
tion ; its lower angle causes a dorsal ridge to project into the 
mouth cavity. Laterally are placed two long rods, which 
are continued into the central rod and notochordal sheath 
anteriorly. To these lateral rods are attached large bunches 
of longitudinal muscles, by which, doubtless, the notochord 
may be pulled backwards, and the proboscis retracted so as to 
shut the mouth (fig. 60). 
Posteriorly the median rod divides into two, and the opening 
from the notochordal lumen into the gut lies in the angle 
formed by the separation of these two diverging rods 
(fig. 57). 
A considerable deposit of ‘ structureless”? substance takes 
place, filing up the spaces in the proboscis stalk, and forming 
a partial sheath around the perihemal cavity. Whether this 
substance is chitinous or of some other material I am unable 
to say. The ensheathing parts of it have exactly the histolo- 
gical appearance preseuted by the “structureless” substance, 
which in Amphioxus is continued from the notochordal sheath, 
8 
