I ALIMENTARY CANAL GILL-SLITS I I 



dorsal groove in the larva, 1 and in a similar manner in the collar 

 which is formed as the result of regeneration after injury. 2 

 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-slits 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 Doliclioglossus 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 3 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, g.s}. Each ordinarily opens 

 to the exterior by a small pore (Fig. 1, D, 5, g.p} or slit, 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. iii. p. 228. 



