CHESTER. — STRUCTUIIE OF I'SEUDOPLEXAURA CRASSA. 747 



Sometimes, ultliougii tlie tentacles are contracted, the mouth remains 

 open and th(> cohnnn is hrouirht down nearly to the level of the eoe- 

 nenchynia, where it remains for a lon^^ time, the current of Mater to 

 and from the stomo(hieum ('vidently not being cheeked (Fig. 1, «). 

 Proi)al)ly the coenenchyma is also capable of slight contraction and 

 expansion, due principally to the action of the muscles of the mesen- 

 teries. When the tip of a branch is cut off, the coenosarc, with the 

 polyps near the line of the cut, becomes appreciably of smaller diam- 

 eter, thus partially covering the cut surface; and the same result is 

 seen in the regenerating branches on the reefs, where the polyps have 

 been turned from their radial position and are held in such a way as to 

 lessen the area of the cut surface. 



Ectoderm. 



The ectoderm of the polyp wall in the expanded condition is more 

 than one cell thick, showing an epithelial and a subepithelial part 

 (Plate 2, Figs. 8, 18). In the ei)ithelial part, the cover cells {cl. leg.) 

 are conical, with a round or polygonal external surface as a base and 

 with the opposite vnd, as apex, extenihng into the mesogloea as a 

 process. In the expanded state of the polyp many of the cells are 

 peltate or nnishroom shaped, since they ha\'e the appearance of an 

 external plate supported by a rapidly narrowing stem. This stem 

 soon takes a thread-like character and may be branched (Figs. 9, 1 1 ). 

 The outer surface is often convex, like a nnishroom, but it may be 

 indented, so that the section of it is cusp like (Fig. 18, cl. leg.). In the 

 expanded condition of the polyp, the plate-like surface covers a 

 relatively large area and the process l)ecomes shorter; in the con- 

 tracted condition, tlie cells approach a cohunnar shape, though the 

 process is still thread-like at the mesogloeal end (Fig. 20, cL teg.). 

 The protoplasm is finely granular and sometimes large gramdes also 

 occur (Figs. 10, 11). Although sometimes without vacuoles, these 

 cells usually contain many small ones. The nuclei are large and 

 gramilar and are situated in the outer half of the cells or near the point 

 of the cusp. Among the covers cells are found nettle-t-ells or thread- 

 cells with their nematocysts (Fig. 18, miicus.) and sense cells (c/. .s7/5.). 



Tlie nematocysts are of two kinds. The larger and more numerous 

 ones (Plate 2, Fig. 18 and Plate 4, Figs. 43-46) occur in batteries of 

 six to ten, but they may be found in much larger numbers. The cyst 

 is sharply outlined and is an elongated oval with a length three times 



