370 



EVENINGS AT THE MICROSCOPE. 



from that already described. It is true I have detected it 

 only in Corynactis, where the short ecthoreum of the 

 Tangled Cnida is surrounded throughout its length by a 

 barbed strebla of three bands. The barbs are visible, 

 under very favourable conditions for observation, even 

 while the tangled wire remains inclosed in the cnida, but 

 their optical expression is that of serratures of the walls, 

 without the least appearance of a screw. This, I say, is 

 the only species in which I have actually seen the arma- 

 ture of the ecthoreum in this kind of cnida, but I infer its 

 existence from analogy in other 

 species, where the conditions that 

 can be recognised agree with those 

 in this, though the excessive attenu- 

 ation of the parts precludes actual 

 observation of the structure in ques- 

 tion. 



Spiral Cnidse constitute the third 

 form. In a few species, as Sagartia 

 parasitica, Tealia crassicornis, and Ce- 

 rianthus membranaceus, I have found 

 very elongated fusiform cnida, which 

 seem composed of a slender cylin- 

 drical thread, coiled into a very close 

 and regular spiral. In some cases 

 the extremities are obtuse, but in 

 others, as in T. crassicornis, an ex- 

 ample of which I now show you, 

 the posterior extremity runs off to a 

 finely-attenuated point, the whole of the spire visible even 

 to the last, the whole bearing no small resemblance to a 

 multispiral shell ; as one of the Cerithiada or Turritellada. 

 The ecthoreum is discharged reluctantly from this form, 

 and I have never seen an example in which the whole had 

 been run off. So exceedingly subtle are the walls of the 

 cnida, that it was not until after many observations that 



CNIDA OF CORYNACTIS. 



