OI.TGOTRKMA PSAMMITES. 253 



detached strips the cell elements were so well preserved that 

 I could study their details with some considerable degree of 

 accuracy. 



The oesophagus is lined by a glandular epithelium, the 

 characters of which are shown in fig. 17. It is made up of 

 columnar cells, with flattened nuclei situated at the extreme 

 basal ends of the cells. The cytoplasm is filled with minute, 

 highly refracting, yellowish granules, probably zymogen 

 granules of some kind. This epithelium is coutinued up the 

 groove on the dorsal side of the branchial sac, and may be 

 traced nearly as far forwai'd as the dorsal tubercle, beyond 

 the point where the groove ceases to be recognisable. 



De Lacaze Duthiers describes a groove which runs forward 

 from the oesophageal opening alongside of the dorsal lamina 

 in Molgula, with which the much larger oesophageal groove 

 of Oligotrema is doubtless homologous ; but I could find no 

 trace of a definite dorsal lamina. 



The epithelium lining the branchial sac and the pre- 

 branchial zone and covering the oral tentacles is of an entirely 

 different character to that of the oesophageal groove, being- 

 composed of elongated cells such as are figured in fig. 18, 

 a, h, c. They are extremely like the collar-cells of sponges, 

 but as I was unable, after long examination, to satisfy myself 

 that their long projecting free ends are, in fact, hollow 

 cylinders with a flagellum passing down the middle of each, 

 I will simply call them modified flagellate cells. These cells, 

 as seen in section, are hammer-shaped ; the head of the 

 hainmer is basal, and the nucleus is situated where the 

 handle is fixed on the head. Adjoining cells are fitted 

 together by their hammer-headed basal ends, and their handles 

 project as long, free processes into the branchial or pre- 

 branchial cavities. The body of the cell (i. e. the head of the 

 hammer) is more granular than the process, and stains with 

 carmine ; the process is stained, but not deeply, by picro- 

 nigrosin or picro-indigo-carmine. In some places the free 

 pi'ocesses are short (fig. 18, c) ; in others they are inordinately 

 long (fig. 18, h). In a considerable number of the cells I 



