Cyathocornius mirabilis n. g., n. sp. 15 



along the dorsal edge of the branchial sac, while the circlet of tenta- 

 cles, the dorsal tubercle, and the peripharygeal band are situated 

 at its anterior extremity (PL II., fig. 8). All these organs will be 

 described further on. Very often one or two small copepod crus- 

 taceans were found living in the branchial cavity as commensals. 



The stigmata occur over the whole extent of the sac, from 

 the peripharyngeal band anteriorly to the base where the oeso- 

 phagus opens posteriorly, with the exception of a narrow band 

 along the dorsal and ventral edges, where the median dorsal vessel 

 and endostyle are placed. They are arranged in horizontal 

 (transverse to the aiitero-posterior axis) rows, and are separated by 

 transverse and longitudinal vessels (PL II., fig. 12). There are 

 four such rows and more than twelve stigmata in a row on each 

 side. In shape they are elongated slits with parallel sides and 

 rounded ends. The width of the stigmata is about equal to that 

 of the fine longitudinal vessels separating them. 



Examined under a moderate power of microscope, each 

 longitudinal vessel is found to have the form of a rectangular 

 pillar, the sides of which are made of two kinds of cells. On the 

 sides facing the branchial and atrial spaces the wall is very thin 

 and is composed of flattened polygonal cells. On the sides sur- 

 rounding the stigmata, on the contrary, the cells are of consi- 

 derable height. Seen from the interior or exterior of the 

 branchial sac each of the stigmata appears to be surrounded by a 

 single row of ciliated cells (PL II., fig. 12). A transverse section of 

 the longitudinal vessels, however, shows most clearly that what 

 looked like a single cell is in reality a group of very narrow cells 

 placed side by side (PI. II. fig. 13), and consequently what 

 appeared as a single row of cells is in reality a longitudinal band 

 of ciliated epithelium extending down the side of the vessel. The 

 ciliated cells vary somewhat in shape, being taller and more 

 columnar at the ends of the stigmata. The most common form is 

 nearly semicirculer, the flat face being next the vessel and the 

 curved one next the stigma. As they are grouped regularly in 

 transverse rows they look somewhat like a pile of coins cut in 

 halves. The nuclei are distinct and placed rather below the 



