THE ANATOMY OP ALOYONIQM DIGITATUM. 365 



The ectoderm is almost invariably covered by a coat of 

 transparent mucus, in which grains of sand, minute algse, and 

 other foreign bodies occur. This mucous coat is probably 

 formed from the secretions of some of the flask-shaped cells. 



With the possible exception of an occasional star-shaped 

 interstitial cell I can find no trace of a ^' Nervenschicht" in 

 the general ectoderm of the colony, nor can I find true mus- 

 cular processes to any of the cells, nor any cnidoblasts or 

 nematocysts. 



The ectoderm-cells of the tentacles are very similar in 

 general appearance to those of the general surface of the colony, 

 but they are not quite so long, i. e. about 0'015 mm. They 

 are prolonged below, however, into well-marked muscular pro- 

 cesses, and between them may be seen numerous cnidoblasts and 

 nematocysts. A nerve-sheath is very distinctly present in the 

 form of minute star-shaped and round ganglion-cells connected 

 with a plexus of very delicate anastomosing fibrils (fig. 22). 



The ectoderm of the oral disc and body-wall is not so easy 

 to investigate, as it becomes detached or partly so with the 

 slightest friction. It seems to vary very considerably in thick- 

 ness according to the condition of expansion of the polyp, 

 but in other respects it does not difi"er in any very marked 

 characters from that of the general surface of the polyp. 



The ectoderm lining the stomodaeum is composed of closely 

 packed columnar cells, 0"017 mm. in length, provided with short 

 cilia (fig. 14i), as in Pennatulids, Clavularia, and A. palmatum 

 (Wilson). The epithelium of the siphonoglyphe diflfers from 

 that of the other parts of the stomodseum in that the cells are 

 narrower, somewhat clearer, considerably deeper (0*025 mm. 

 in length), and bear long (0*025 mm.) whip-like flagella 

 (fig. 29). 



The nematocysts of Alcyonium are extremely small (0'0075 

 mm.), and all of one kind. They may easily be seen by 

 snipping ofi" a tentacle of an expanded polyp and examining it 

 with a microscope, when, on the addition of a little acid, they 

 may be seen to shoot out their threads, and occasionally get 

 loose from the ectoderm and float oflf freely in the surrounding 



