138 JOURNAL OF THE [J"ly> 



coelenterate animals were described as wholly smooth, which are 

 now known to be clothed with very fine hairs, and that others 

 were said to be merely serrate, which better lenses now prove to 

 be setaceous. If the largest threads are to be taken as typical 

 of the whole class, the general disposition of the setae upon the 

 thread is in spiral lines. According to Mr. Gosse's description 

 of the ecthoraeum of Tealia crassicomis, " the screw is formed 

 of a single band, having an inclination of 45° to the axis;" 

 while in Sagartia parasitica " we find a screw of two equidis- 

 tant bands, * * * having an inclination of 70°;" and " in 

 Cyathina S/nithii, the strebia is composed of three equidistant 

 bands, * * * ^yith an inclination of about 40° from the 

 axis." The threads from the largest cells of our Isophyllia 

 (shown in Fig. IV.) correspond with those last described by Mr. 

 Gosse, in respect to the number of the spiral lines, as I suppose 

 we might expect from the relationship of the two genera. 



It has generally been assumed that the setae serve the same 

 purpose as does the barb of an arrow or a spear, — namely, of 

 fixing the weapon in its wound. But this inference is somewhat 

 weakened by the fact, in the first place, that many organisms 

 which possess thread-cells manifest very little or no power of 

 stinging ; by the further fact that the same organism is often 

 provided with several different kinds of thread-cells, apparently 

 adapted to more than one ofifice, and sometimes located in parts 

 of the animal which cannot be brought into use offensively; 

 and, finally, by the fact that the setae are not always situated 

 upon a part of the thread likely to penetrate, or which would 

 even reach, the integument of an animal attacked, — an exception 

 which is exemplified by the narrow band of long hairs placed 

 near the base of the filament of the smallest cells of Isophyllia, 

 which are confined to a space of only about ^fJxrth of an inch, 

 from which to its tip the thread appears to be smooth. Taking 

 all these things into account, and considering also the analogies 

 of the case and the collateral circumstances to which I have 

 already referred, I am of the opinion that the main purpose of 

 the setae, instead of being similar to that of the arrow-barb, 

 is rather like that of the "feather" on the dart of the air-gun, 

 or, perhaps more exactly, like that of the piston-head in the 

 cylinder of the steam engine; — for I take it that the setae fill out 

 the neck-like tube of the nematocyst and receive the impact of 

 the propelling force applied from within the cell. 



