'I'HE MORPHOLOGY OF THE CHEII.OSTOMATA. 269 



tery " (fig. 22, m.). The effect of the arrang-oment indicated 

 in fig-. 27 is to keep the epitheca stretched as a flat raembraue 

 at some distance from the basal calcareous walls of the 

 zooecia. The cavity beneath the epitheca is divided into a 

 series of parallel longitudinal spaces by the parts of the 

 zooecia above described. The linear figure formed by the 

 insertion of the zooecium into the basal epitheca is in some 

 cases bifurcated proximall}^ On the frontal surface (figs. 

 20, 21) each orifice is surrounded by a soniewliat irregular 

 ring of calcareous matter, from each side of which is given 

 off a sti'oug condyle (fig. 21, cond.) or " denticle," the two 

 condyles forming the hinge of the operculum. The fi'ontal 

 surface is strengthened by a highly variable arrangement of 

 calcareous bars, the general position of which is shown in 

 fig. 20. The bars are in the main flattened, their flat surfaces 

 being parallel with the surface of the bi'anch, but in curving 

 down into the lateral walls they usually give off vertical 

 flanges from their free surface, and these form bridges across 

 the depressed intervals between two zooecia, joining similar 

 flanges in the adjacent zooecia. At the free end of the 

 brauch the proximal parts of the bars are first formed, and 

 they grow in a distal direction beneath the epitheca. 



Along the lateral margin of the branch runs a tube (fig. 

 20, m.c.)^ which has usually been described as a ^'chitinous 

 fibre" in other forms of Flustrine habit. This is merely a 

 part of the branch wlucii is not divided into zooecia, and 

 calcareous bars (c. h.) extend from the marginal zooecia nearly 

 to the outer edge of its free surface; it contains, moreover, 

 strands of funicular tissue which pass across its lumen. This 

 space runs as a continuous tube along the whole margin of 

 the brauch, and it communicates with the cavity which lies 

 between the backs of the marginal zooecia and the basal 

 epitheca. 



E. clathrata, like E. obtecta, is characterised by the 

 dimorphism of its opercula. This is shown in figs. 20, 21, 

 representing the ordinary type ("a") and the second form 

 ' Cf. Waters, 18'J6, p. 291. 



