80 BALANOGLOSSIDA. 



The coelom, though largely filled up by muscular and con- 

 nective tissues in the adult, presents the arrangement which, as 

 we have seen in the first chapter of the second volume (p. 7), 

 is on the whole characteristic of the Chordata (see Fig. 72). .It 

 is present in three distinct parts which remain separate from one 

 another : these are the unpaired preoral or proboscideal sac, the 

 paired collar sacs, and the paired trunk sacs. In the embryo 

 these sacs all contain distinct cavities and have epithelial 

 walls (Figs. 72, 73). In the adult the walls have increased in 

 thickness and have become differentiated into the muscular and 

 connective tissues of the body ; while the cavities are much 

 reduced, being encroached upon and partly obliterated by these 

 tissues. 



The coelom of the proboscis. The wall of the coelomic sac 

 of the proboscis consists of a thin layer of circular muscles lying 

 next the basement membrane of the ectoderm and inside this 

 of a thick layer of longitudinal muscles, which are often arranged 

 in bands and which surround the central cavity (Fig. 65). 

 Within the longitudinal muscles and extending amongst them is 

 a layer of loose stellate connective tissue which partly fills up the 

 proboscis anteriorly. 



Into the posterior region of the proboscis cavity there projects 

 a complicated structure known as the central complex or the 

 basal organ of the proboscis (Figs. 60, 64). The central complex, 

 which consists of the notochord and certain vascular organs, is 

 almost entirely covered towards the proboscis cavity by a layer 

 of coelomic epithelium, and is attached to the body wall in the 

 dorsal and ventral middle lines by the dorsal and ventral septa 

 of the proboscis (Fig. 65, I, 6). The posterior part of the pro- 

 boscis cavity is therefore double and lined, at any rate posteriorly 

 in the region of the dorsal and ventral canals (see below), with 

 coelomic epithelium ; while the anterior part (solid in Harrimania 

 kupfferi) is single (except in Willeyia in which it is double) and 

 without, so far as can be seen, a distinct layer of coelomic epithe- 

 lium. Each of the posterior halves of the proboscis cavity is 

 further subdivided, in consequence of lateral expansions of the 

 notochord which meet the body-wall, into a dorsal and a 

 ventral portion (Fig. 66).* In this way are formed the two 

 dorsal and the two ventral canals of the proboscis coelom. 

 * In Fig. 66 the two ventral canals are joined, see below. 



