OLIGOTEEMA PSAMMITES. 251 



cavity, and still further forwai'd tlie ventral suspensory fold 

 is couspicuously thicker and contains a considerable centi'al 

 cavity, into which a number of irregular lateral chambers 

 open. These in turn communicate by slit-shaped stigmata 

 with the right and left peribrauchial cavities. Fig. 10 is a 

 drawing of a section taken still further forward, and shows 

 that the cavity in the ventral suspensory fold — which cavity 

 we may recognise henceforth as the branchial sac — has 

 increased greatly in extent and communicates by a narrow 

 passage with the oesophagus. The right peribrauchial cavity 

 retains its size, but the left cavity is notably diminished 

 owing to the dorsal extension of the left corner of the bran- 

 chial sac. Fig. 9 represents a section taken through the 

 middle of the branchial sac. It shows that the ventral part 

 of the branchial sac extends widely to the right and left, 

 and that there is a corresponding reduction of the two peri- 

 branchial cavities. The cavity of the branchial sac communi- 

 cates on each side by wider or narrower orifices with a 

 number of secondary cavities, which may be called exhalent 

 chambers. These open in turn by numerous stigmatic slits 

 into the right and left peribranchial cavities. The oesophagus 

 communicates freely with the branchial sac, but is still 

 distinct from it, and, as will be shown directly, is lined by a 

 different kind of epithelium. 



Fig. 8 is a section through the upper end of the branchial 

 sac. The peribranchial cavities are reduced to small dimen- 

 sions ; the cavity of the branchial sac has increased very 

 much in size, and the oesophagus is reduced to a deep groove 

 on its dorsal surface ; but the epithelium lining the groove is 

 still sharply marked off by its histological characters from 

 that of the branchial sac proper. 



Still further forward, at the extreme anterior end of the 

 branchial sac, tlie peribranchial cavities are reduced to mere 

 canals, into which a few stigmata open. The groove lined 

 by oesophageal epithelium may be traced as far forward as 

 the prebranchial zone, and there it dies out altogether. 



It should be observed that the anterior extensions of the 



