88 



BALANOGLOSSIDA. 



which are forward extensions of the trunk coelom and will be 

 described later. 



The collar canals are two short tubes lined by a ciliated epithe 

 lium, and placing the collar-coelom in communication with the 

 exterior. They occur dorso-laterally at the hind end of the 

 collar, and they open externally by the collar pores into the first 

 branchial pouch. Internally they open by a funnel-shaped 

 aperture into the collar coelom, which, in the neighbourhood 



. of the funnel, is free 

 from the tissue ele- 

 ments which pervade 

 it elsewhere. The col- 

 lar canals are covered 

 towards the coelom 

 by a layer of flat 

 epithelium, and their 

 dorsal walls are often 

 infolded so as to pro- 

 ject into the cavity 

 of the canal.* 



The visceral wall of 

 the collar coelom pre- 

 sents along a certain 

 line on each side a 

 fold containing blood 

 vessels and called the 

 vascular fold. In the 

 Ptychoderidae the 



---8 



l<J. 69. Diagram of a transverse section through collar 

 region of a Balanoglossid to show the relations of the 

 body-cavities in the collar (from MacBride). The coelo- 

 mic spaces are represented as clear of tissue and the 

 ventral mesentery as persistent. 1 medullary nerve 

 cord ; 2 dorsal blood vessel ; 3 perihaemal cavity ; 4 

 notochord at its point of attachment to the buccal wall ; 

 5 posterior diverging crura of the proboscis skeleton ; 6 

 collar coelom ; 7 buccal cavitv ; 8 peripharvngeal cavity : 

 9 ventral vessel. 



vascular fold begins 



at the hind end of the collar region and extends along the ventral 

 surface of the alimentary canal in the middle line to a point not 

 far removed from the anterior end of the collar. Here it divides 

 into two folds which pass transversely one on each side to the 

 dorsal surface. In the other genera the fold is double throughout 

 its course. It begins at the hind end of the collar ventrally and 

 ascends obliquely dorsalwards and forwards on each side to the 

 point where the neck of the proboscis joins the body. The 

 vascular folds contain blood vessels which unite the median 



* This fold recalls a similar structure projecting into the sand canal of 

 some Echmoderms. 



