SEA- ANEMONES : THEIR WEAPONS. 



867 



are invariably furnished with a peculiar armour. The 

 basal portion, for a length equal to that of the cnida, or 

 a little more, is distinctly swollen, but at the point indi- 

 cated it becomes (often abruptly) at- 

 tenuated, and runs on for the remainder 

 of its length as an excessively slender 

 wire of equal diameter throughout. In 

 the short ecthorea of Sagartia the atte- 

 nuated portion is obsolete. 



It is chiefly upon this ventricose or 

 swollen basal portion that the elaborate 

 armour is seen, which is so characteristic 

 of these remarkable organs. For around 

 its exterior wind one or more spiral 

 thickened bands, varying in different 

 species as to their number, the number 

 of volutions made by each, and the 

 angle which the spiral forms with the 

 axis of the ecthoreum. The whole spiral 

 formed of these thickened bands is 

 termed the screw, or strebla. 



In the ecthorea emitted by chambered 

 cnida from the craspeda of Tealia eras- 

 sicornis, the screw is formed of a single 

 band, having an inclination of 45° to the 

 axis, and becoming invisible when it has 

 made seven volutions. In those from 

 the same organ in S. parasitica, we find 

 the Bcrew of two equidistant bands, 

 each of which makes about six turns — 

 twelve in all — having an inclination of 70° from the com- 

 mon axis. In those similarly placed in Caryophyllia 

 Smithii [now under your observation], the strebla is com- 

 posed, as you may perceive, of three equidistant bands, 

 each of which makes about ten turns — thirty in all — 

 with an inclination of about 40° from the axis. In every 



CNIDA OF 

 TEALIA CRASSICOENIS. 



