Dr Grant mi the Structure and 2'Zature qfFlustrcB. 339 



tremities of the last two rows of cells ; this remarkable difference 

 is observable by the aid of a common lens. 



The cells are shorter and proportionally broader than in the 

 F. carhasea, being about the sixth of a line in length, and a little 

 more than half as much in breadth. By tearing the two plains 

 of cells asunder, we render them nearly as transparent as in the 

 F. carbasea, and can distinctly perceive the structure of the 

 parts within. The aperture of the cells is formed by a semicir- 

 cular lid, convex externally and concave internally, which folds 

 down when the polypus is about to advance from the cell. The 

 opening of this lid in the F. truncata, where it is very long, ap- 

 pears through the microscope like the opening of a snake^s jaws, 

 and the organs by which this motion is effected are not percep- 

 tible. The lid of the cells opens and shuts in JlustrcB^ without 

 the slightest perceptible synchronous motion of the polypi. We 

 sometimes observe parts, in other calcareous zoophytes, possessing 

 distinct power of motion, though apparently unconnected with 

 the body of the polypi ; thus in the Cellaria avicularia, Lam. 

 whose polypi have the same structure, and the same connection 

 with the cells as mflustrce, I have observed in living specimens 

 a constant motion of flexion and extension in the remarkable 

 testaceous processes shaped like a bird's head, and attached by pe- 

 duncles to the outside of all the cells. These processes or organs 

 are likewise provided with lateral folds, like the valves of a 

 shell, which have a distinct and regular motion corresponding 

 with the flexion and extension of the entire process. The aper- 

 ture of each cell of the F. foliacea, is defended by four project- 

 ing spines, which arise from the calcareous margin of the cell. 

 There are two spines on each side of the aperture, and the upper 

 two are more than twice the length of the lower pair, and slight- 

 ly curved upwards. When we look transversely on the surface 

 of a branch, the spines appear to be arranged in very regular 

 transverse curved rows, and when we observe the surface longi- 

 tudinally, they appear to be arranged in very regular longi- 

 tudinal straight lines. The spines are calcareous, tubular, 

 cylindrical, shut at their extremity, and appear obviously destin- 

 ed to protect the expanded polypi. The two pairs of spines 

 belonging to each cell are placed only on the upper half of the 

 cell, although, from the contiguity of the cells, the lower half 

 of each is likewise defended by the spines of the adjacent cells. 



