HYDRACTINIA ECHINATA. 25 



the crust, the polypites are of much smaller size ; aucl in 

 this region occur the tentacular filaments, which are 

 capable of great extension, and float like long fishing- 

 lines through the water. When not extended, these are 

 so inconspicuous as readily to escape observation. Rising 

 amongst the dense ranks of the polypites is a multitude of 

 serrated spines ; and supporting the whole array of curious 

 and beautiful structures spreads the common crust with 

 its soft overlying carpet of coenosarc. 



A remarkable point in the history of the Hydractinia is 

 the amount of sympathy that exists between the zooids 

 composing a colony. This is due to the horizontal fleshy 

 layer which immediately connects them all. It has been 

 noticed that if t any part of the common base be irritated 

 the spiral appendages uncoil simultaneously, and lash 

 themselves violently backwards and forwards, and then 

 quickly roll themselves up again *. I have seen a whole 

 company discharge themselves with remarkable energy, 

 and with the precision of a regiment on drill. 



After studying the structure and the singular associated 

 movements and the constant position of these bodies, I 

 have no doubt that Dr. Wright is correct in regarding 

 them as special organs of the zoophyte (" forms of a 

 truly definite nature"), notwithstanding the opposite de- 

 cision of Agassiz. 



It is difficult to assign them a function, unless they be, 

 as Dr. Wright has conjectured, " organs of defence or 

 offence/ 5 They may, perhaps, be analogous in this 

 respect to the nematophores of the Plumulariida or the 

 curious tentacular appendages of Ophiodes. 



* Vide a very interesting paper on Hydractinia in the Edinburgh New 

 Phil. Journ. for April 1857, by Dr. Strethill Wright, to whom we are in- 

 debted for the first notice of the spiral and tentacular appendages, and a very 

 accurate and philosophical account of the general structure of this zoophyte. 



