OLIGOTREMA PSAMMlTES. 249 



The Branchial Siphon, Branchial Sac, and Gut. 



The general relations of these organs and of the other 

 viscera associated with thein are shown in figs. 3 and o4, and 

 in the series of sections, figs. G to 15. The branchial aperture 

 has the form of a transversely elongated slit, and the branchial 

 siphon, lined by a thick involution of the test, is also com- 

 pressed in the transverse plane. The mouth at the bottom 

 of the branchial siphon is not a circular orifice, but is narrow 

 and slit-like, and slants dorso-ventrally owing to the branchial 

 siphon being produced backwards on the ventral side to form 

 a sort of half tube or gutter, crescentic in section. The mouth 

 is surrounded by a circle of small, ramified, oral tentacles. I 

 was unable to determine the number of these tentacles from 

 my sections, but they are fairly numerous. I should judge 

 that there are sixteen or more, and they are distinctly, but 

 not largely, ramified. They are covered by a peculiar, modi- 

 fied, flagellate epithelium, which also lines the prebranchial 

 zone and a large part of the branchial sac, and will be 

 described in detail further on. 



The branchial sac is one of the most remarkable features of 

 Oligotrema, both because of its small size and because of the 

 great reduction of the characteristic structures associated with 

 it, such as the endostyle. The latter structure, indeed, is so 

 feebly developed that no trace of it could be seen through 

 the transparent walls of the body, even after the specimen 

 had been stained with borax carmine and cleared in xylol 

 preparatory to being embedded in paraffin. Under the same 

 conditions the branchial sac itself could hardly be recognised, 

 but it could be discovered, on close observation, as a dilata- 

 tion of the anterior end of the oesophagus (fig. 3, Br. s.). 

 Its extent and relations to the branchial siphon and adjacent 

 organs are clearly shown in fig. 34. The oesophagus is a 

 relatively wide tube running back on the dorsal side; its 

 length is about one third of the whole length of the body, 

 and it is therefore a much more important and conspicuous 



