196 J. H. ASHWORTH. 



this stage the spicule grows regularly in length and thickness, and the 

 protoplasm of the cell covering it becomes gradually thinner until, in a 

 fully formed spicule, this protoplasmic sheath forms an exceedingly 

 thin investment, in which at one part may be seen the small, some- 

 what flattened nucleus embedded in a small mass of protoplasm (Figs. 

 13 and 14). In all the specimens, many hundreds in number, which I 

 have examined, the spicule develops in a single cell with one nucleus. 

 I have not been able to find any examples which showed that two cells 

 or two nuclei were concerned in the formation of the spicule (cf, v. 

 Koch's account of the development of the spicules of Clavularia j)ro- 

 lifera, ' Morph. Jahrb.,' vii, p. 473, 1882). 



Mesenteries (PI. XXI). 



Mesenterial Filaments. — The stomodeeum leads into the coelenteron 

 of the polyp, which is subdivided by the usual eight mesenteries (PI. 

 XX, Fig. 10). Of these only the two dorsal ones possess thickened 

 edges or mesenterial filaments (PL XXI., Fig. 19). The free edge of 

 the remaining six mesenteries is only very slightly thickened, this 

 being due entirely to the presence of a slightly greater amount of 

 mesogloea near the free edge of the mesentery. The cells which cover 

 this thickened portion differ in no way from those covering the other 

 parts of the mesentery (PI. XXL, Fig. 20). The six ventral and lateral 

 mesenterial filaments usually present in the polyps of the Alcyonaria 

 are not found in this genus. The dorsal mesenterial filaments arise 

 from the lower edge of the stomodeeum and run in a sinuous course 

 along the dorsal side of the coelenteron. In the primary polyps they 

 may be traced to the base of the colony. In transverse section the 

 filament is slightly bilobed, i.e., there is a groove (of slightly varying 

 depth) extending all the way down the middle of the free surface of 

 the filament (Fig. 19). The cells on each side of this gi'oove bear long- 

 cilia ('0075 mm.). The dorsal mesenterial filaments are quite typical, 

 and agree well with the accounts given of those of other A Icyonaria by 

 Wilson and Hickson. They are probably ectodermic in origin, as they 

 appear to be formed as two downgrowths from the inner end of the 

 stomodseum. The cells of the filaments agree in structure with those 

 of the stomodaeum, being finely granular and non-A r acuolated, and differ- 

 ing markedly from the much vacuolated neighbouring endoderm cells. 



