8 HEMICHORDATA CHAP. 
over the surface of the proboscis. At first the animal made slow 
progress ; but the collar, becoming surrounded by sand, soon 
became a point of resistance by means of which the proboscis 
could bury itself yet more deeply. The animal quickly disap- 
peared as soon as the first two regions of its body were engaged 
in the task of burrowing.’ 
This action is due partly to the muscles of the body-wall, but 
largely to the power possessed by the proboscis and collar of 
becoming swollen and turgid. Spengel has observed that these 
parts become flaccid when the animal is taken out of water, and 
can only swell again when it is replaced therein; and it may thus 
fairly be concluded that the enlargement is due to the taking in 
of water. This is probably in fact the most important function 
of the “ proboscis-pore ” and of the “collar-pores” which are de- 
scribed below. 
Body-Cavities.—The existence of five separate body-cavities 
(Fig. 2) is one of the most fundamental facts in the anatomy of 
Balanoglossus. The first body-cavity, or 
cavity of the proboscis (d.c'), is single 
and unpaired; the second body-cavities 
(b.c”) are paired spaces, one belonging to 
each side of the collar; the third body- 
cavities (b.c*) are similarly paired, and 
correspond with the trunk. While there 
is no connexion between successive body- 
cavities, there are in certain regions com- 
munications between the two cavities of 
Fic. 2.—Diagram of a dorsal the same pair. Hach of the paired 
view of a Balanoglossus- cavities is at one time a closed lateral 
embryo, after the forma- : ‘ 
space between the skin and the alimentary 
tion of the body-cavities. 
a, Alimentary canal; 0.c’, canal. As the two spaces which con- 
body-cavity of the pro- : ; 
boscis ; 4.c2, of the collar; Stitute the pair grow towards one another, 
ian trunk, (From oth above and below the alimentary 
canal, they come into such close apposi- 
tion that they remain separated only by their conjoined walls. 
In this way are formed the dorsal and ventral mesenteries (Fig. 
4, d.m,v), the former being the only one to persist in the higher 
Vertebrates. The body-cavities of the adult become to a large ex- 
tent disguised by being traversed by connective tissue and muscles. 
1 See also Ritter, Biol. Bull. iii, 1902, p. 255. 
