PROBOSCIS COELOM. 



83 



of the pericardium,* in the region anterior to the dilated part 

 of the notochord, where the dorsal canals acquire their coelomic 

 epithelial lining. f 



The extension of the ventral septum and of the median muscle 

 septum of the proboscis exhibits considerable diversity in the 

 group. 



It is now necessary to describe more in detail the posterior 

 more definite portions of the proboscis coelom, where an epithelial 

 lining is pre- 

 sent. First of 

 all the ventral 

 canals; they 

 nearly always 

 join posteriorly 

 to form the 

 ventral ccecum 

 which extends 

 back to the root 

 of the proboscis 

 (Fig. 66, ehv), 

 and the hind 

 end of which in 



eh--- 



reh 



ehv?- 



FIG. 66. Transverse section through the neck of the proboscis of 

 Balanoglossus gigas behind the ventral septum (after Spengel). eh 

 dorsal canal of coelom, leading on the left side to the pore ; h tissue 

 in the pericardium ; div notochord (buccal diverticulum) ; sk pro- 

 boscis skeleton; ehv hind end of ventral canal (ventral caecum). 

 The efferent vessels of the proboscis are shown on each side dorso- 

 laterally of the notochord. 



some species 

 forms a small 

 projection (the 

 racemose or- 

 gan) from the 

 roof of the buc- 

 cal cavity. 



The dorsal 



canals are more complicated. They pass backwards in the neck 

 of the proboscis as narrow canals lined with a ciliated epithe- 

 lium (Fig. 66 eh), and they either end blindly or open, at the 

 hind end of the neck, into terminal bladder-like dilatations. 

 These two end-vesicles open to the exterior by the proboscis 

 pores. As a rule there is only one proboscis pore and end- 

 vesicle, viz. on the left side, but when there are two, those of the 



* It appears to have done this in the section from which Fig. 65 was 

 taken. 



f It will be remembered that anteriorly the proboscis coelom appears to 

 be without epithelium in the adult (see p. 80). 



