HEMICHORDATA 



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. 1 



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 (b.c 1 ), is single 

 and unpaired ; the second body-cavities 

 (&.c 2 ) are paired spaces, one belonging to 

 each side of the collar ; the third body- 

 cavities (6.c 3 ) 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 



FIG. 2.-Diagram of a dorsal the S ame P air - Each of tlie P&ired 



view of a Baiauogiossus- cavities is at one time a closed lateral 



embrj'o, after the forma- . , , . , , , , . 



tion of the body-cavities, space between the skin and the alimentary 



a, Alimentary canal; b. c l , cana l. ^8 the two Spaces which COn- 

 body-cavity of the pro- ... 



boscis ; b.c 2 , of the collar ; stitute the pair grow towards one another, 

 5.C 3 , of the trunk. (From ^ Q ^ a bove and below the alimentary 



Bateson.) 



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. 



