INTRODUCTION. XXXI 



When fully expellee!, the thread or wire, which I distin- 

 guish by the term ecthorceum (Plate XI. fig. 7, n), is often 

 twenty, thirty, or even forty times the length of the cnida ; 

 though, in some species, as in most of the Sagartice, it 

 frequently will not exceed one-and-a-half, or two times the 

 length of the cnida. 



The ecthorcea, which are discharged by chambered cnidce, 

 are invariably furnished with a peculiar armature. The 

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

 little more, is distinctly 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 diameter throughout. In the short ecthorcea of 

 Sagartia, the attenuated portion is obsolete. 



It is chiefly upon this ventricose basal portion that the 

 elaborate armature 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 ecthorceum. The whole spiral, formed of 

 these thickened bands, I designate the screw, or strebla 

 (%• 7,o). 



In the ecthorcea emitted by chambered cnida from the 

 craspeda of T. crassicornis, the screw is formed of a single 

 band, having an inclination of 45° to the axis, and be- 

 coming invisible when it has made seven volutions. In 

 those from the same organ in S. parasitica 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 

 Garyophyllia, the strebla is composed 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 north, 

 supposing the axis to point perpendicularly upwards. 



Sometimes, especially after having been expelled for 

 some time, the wall of the ecthorceum becomes so attenu- 

 ated as to be evanescent, while the strebla is still distinctly 

 visible. An inexperienced observer would be liable, under 

 rfuch circumstances, to suppose that the screw, when formed 

 of a single band, as in T. crassicornis, is itself the wire; 

 an error into which I myself had formerly fallen. An 



