r ALIMENTARY CANAL—GILL-SLITS It 
dorsal groove in the larva,’ and in a similar manner in the collar 
which is formed as the result of regeneration after injury.” 
Balanoglossus is thus typically provided with a dorsal, tubular, 
central nervous system, and although this arrangement does not 
extend beyond the limits of the collar, it shows a noteworthy 
resemblance to Vertebrate animals. 
In some cases the central nervous system is connected with 
the dorsal epidermis by a varying number (1-17) of median 
“roots,” which have been compared by Bateson with the dorsal 
roots of the spinal nerves of Amphioxus, and are probably 
remains of the embryonic connexion of the collar nervous system 
with the dorsal epidermis. 
Alimentary Canal.—The mouth (Fig. 7, m) leads widely into 
the alimentary canal, which, passing through the collar, enters 
the branchial region, where it is characterised by the existence 
of communications with the exterior. These, the gill-slits, are 
developed, as in Vertebrates, as paired outgrowths of the 
alimentary canal, and new gill-shts are constantly being formed 
at the posterior end of the branchial region with advancing age. 
The maximum number of the gill-slits, and the extent of the 
branchial region, are by no means uniform throughout the 
Enteropneusta. Thus Dolichoglossus otagoensis is said to have 
no more than 12 pairs, Glossobalanus minutus only 40 pairs, 
while Balanoglossus aurantiacus may have as many as 700 pairs. 
In Ptychodera flava the variation is so great that Willey dis- 
tinguishes* two extreme conditions as “macrobranchiate” and 
“ brachybranchiate ” respectively, although intermediate con- 
ditions are also found. It should be noted that Balanoglossus 
agrees with Amphioxus in the indefinite number of the gill- 
slits. 
The gill-slits usually have the form of the so-called “ branchial 
pouches” or “ gill-sacs” (Figs. 5, 6, 4.8). Each ordinarily opens 
to the exterior by a small pore (Fig. 1, D, 5, g.p) or shit, situated 
on the dorsal side, in a shallow longitudinal groove not far from 
the middle line. The gill-sac has a complete wall of its own, 
and lies between the alimentary canal and the body-wall, com- 
municating with the former by a U-shaped slit. While a dorsal 
1 Morgan, J. Morphol. v. 1891, p. 422; ix. 1894, pp. 44, 48, 72. 
2 Willey, Zool. Res. Pt. iii. 1899, p. 245. 
3 Zool. Res. Pt. ili. p. 228. 
