XIII 



PHYLUM CHORDATA 



are mature. With the collar region are connected a series of twelve 

 arms or tentacles, each beset with numerous very fine filaments 

 and containing a prolongation of the collar cavity. The proboscis 

 (Fig. 671, ps.) is a shield-shaped lobe overhanging the mouth ; its 

 cavity communicates with the exterior by two proboscis pores (p.p). 

 The cavity of the collar communicates with the exterior by a pair 

 of ciliated passages opening by the collar pores. Behind the collar 

 region is on each side a small area in which the body- wall and 

 that of the pharynx are coalescent ; this area is usually, though 



VIG. 671. Cephalodiscus. Diagram of longitudinal section, a. anus ; &C 1 . column of pro- 

 boscis ; &c 2 . coelom of collar ; bc$. ccelom of trunk ; int. intestine ; nch. supposed notochord ; 

 n. s. nerve-strand ; oes. oesophagus ; ov. ovary ; ovd. oviduct ; ph. pharnyx ; p. p. proboscis 

 pore ; ps. proboscis ; st. stomach ; stk. stalk. (After Harmer.) 



not always, perforated by an opening the gill-slit. A nerve- 

 strand containing nerve fibres and ganglion cells is situated on 

 the dorsal side of the collar and is prolonged on to the dorsal sur- 

 face of the proboscis and the dorsal surface of the arms. On the 

 ventral side of this nerve-strand is a very slender cylindrical 

 cellular cord (nch.) continuous behind with the epithelium of the 

 pharynx : this is supposed to represent the diverticulum of Bala- 

 noglossus, and thus to be homologous with the nofcochord of the 

 Ohordata. The posterior end of the body is drawn out into a sort 

 of stalk on which the buds are developed (Fig 1 . 670). A pair of 



