78 



BALAJSTOGLOSSIDA. 



The walls of the pharynx are supported by a definite skeleton 

 recalling the branchial skeleton of Amphioxus (see p. 91). 



The gill-slits of the two sides are separated from one another 

 dorsally by a narrow streak of endoderm called the epibranchial 

 streak (Fig. 63, 1). Their ventral extension is more variable. In 

 Schizocardium they almost meet ventrally, being separated only 

 by a narrow band of endoderm which is called the hypobranchial 

 streak (Fig. 63, C). In Dolichoglossus and Glandiceps they 

 extend half way down the pharyngeal wall, being separated by 

 a wide tract of continuous endoderm, which may also be called 

 the hypobranchial streak (Fig. 63, A and B). 



Lastly in the Ptychoderidie and in Spengelia. they only extend 

 half way down the pharyngeal wall, and the hypobranchial 

 portion is wide as it is in Dolichoglossus ; but at the junction of 



hypobranchial streak (from MacBride). 



the two parts the lining of the pharynx is much thickened, form- 

 ing the so-called parabranchial ridges (Fig. 61, 13} and its lumen 

 is constricted in such a way that the whole pharynx has in 

 section a figure-of-8 shape. The result of this arrangement is 

 that the pharynx is almost divided into two tubes a dorsal 

 tube in connexion with the gill-slits and a ventral tube com- 

 parable to the hypobranchial streak of other forms. In 

 ffldiidiceps two conditions are found ; in some species the arrange- 

 ment resembles that found in Dolichoglossus, in others the 

 condition described for Schizocardium is repeated. 



It seems obvious to compare the epibranchial streak with the hyper- 

 l>liupyngeal groove of Amphioxus and the hypobranchial which is so 

 much developed in Ptychodera and Spengelia with the hypopharyngeal 

 irroove or endostyle* of that form. The pharynx sometimes retains its 



* The term oesophagus is sometimes applied to the ventral portion of 

 the pharynx, but this is obviously a misnomer and must be rejected. 



