SEA-AXEMONES '. THEIR WEAPONS. 421 



equal to that of the cnida, or a little more, is distinct!/ 

 swollen, but at the point indicated it becomes (often 

 abruptly) attenuated, and runs on for the remainder of 

 its length as an excessively slender wire of equal diam- 

 eter throughout. In the short ecthoraea of Sagartia^ 

 the attenuated portion is obsolete. 



" It is chiefly upon this ventricose basal portion 

 tliat the elaborate armature is seen, which is so char- 

 acteristic 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 tlie axis of the ectJioi'deum. 

 The whole spiral formed of these thickened bands, is 

 termed the screw, or strebla. 



" In the ecthorma emitted by chambered cnidce frjm 

 the craspeda of Tealir crassicornis^ the screw is formed 

 of a single band, having an inclination of 4-5" to the 

 axis, and becoming invisible when it has made seven 

 volutions. In those from tlie same organ in S. jpara- 

 sitica^ we find a screw of two equidistant bands, each 

 of which makes about six turns — twelve in all — having 

 an inclination of 70° from the common axis. In those 

 similarly placed in Cyathina Sinithii^ [now under your 

 observation,] the strebla is composed, as you may per- 

 ceive, of three equidistant bands, each of which makes 

 about ten volutions — thirty in all — with an inclination 

 of about 40° from the axis. In every case the spiral 

 runs from the east towards the nnrth, supposing the 

 axis to point perpendicularly upwards. 



" Sometimes, especially after having been expelled 

 for some time, the wall of the cctJiorceinn becomes so 

 attenuated as to be evanescent, while the strebla is still 



