106 



PHYLUM ENTEROPNEUSTA. 



preoral lobe (Figs. 77, 2, 78, 18) called the buccal disc or proboscis. 

 Behind this there is a region of the body which, except in the 

 young bud, is not very distinctly marked off either from the 

 proboscis in front or from the trunk behind. This is the collar 

 region. 



16 



IS 



FIG. 78. Median longitudinal vertical section through Cephalodiscus dodecalophus (after 

 Harmer, from Lang). The figure is diagrammatic, especially in the fact that certain struc- 

 tures not in the middle line are shown. 2 nervous system ; 3 collar coelom ; 5 postoral 

 lamella (operculum) ; 8 trunk coelom ; 9 pharynx ; 10 oesophagus ; 11 stomach ; 12 in- 

 testine ; 13 buccal cavity ; 14 pedicle ; 15 ovary ; 16 anus ; 17 oviduct ; IS proboscis 

 (buccal shield) ; 19 proboscis coelom ; 20 one of the proboscis pores ; 21 notochordal diver- 

 ticulum of the pharynx. 



The collar forms no projection round the base of the proboscis 

 in the mid-dorsal region, but its ventral and lateral portions pro- 

 ject forward, forming a kind of lip (Figs. 78, 79). This lip is called 

 the operculum* : it forms the ventral and lateral edge of the 



* The operculum can be folded backwards, a peculiarity which com- 

 bined with the inconspicuousness of the ventral part of the collar has led 

 some authors to describe it as a projection of the posterior edge of the 

 collar. 



