416 



EVENINGS AT THE MICROSCOPE 



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

 it only in Corynactis, where the short ecthorceum of the 

 Tangled Cnida is surrounded throughout its length by a 

 barbed strebla of three bands. The barbs are visible, under 

 very favorable conditions for observation, even while the 

 tangled wire remains enclosed 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 armature of the 

 ecthorceum in this kind of cnidce, but I 

 infer its existence from analogy in other 

 species, where the conditions that can be 

 recognized agree with those in this, though 

 the excessive attenuation of the parts pre- 

 cludes actual observation of the structure 

 in question. 



4 * Spiral Cnidae constitute the third form. 

 In a few species, as Sagartia parasitica, 

 Tealia crassicornis, and Cerianihus mem- 

 branaceus, I have .found very elongated 

 fusiform cnidce, which seem composed of 

 a slender cylindrical thread, coiled into 

 a very close and regular spiral. In some 

 cases the extremities are obtuse, but in others, as in 

 T. crassicornis [an example of which I now show you], 

 the posterior extremity runs oS to a finely attenuated 

 point, the whole of the spire visible even to the last, the 

 whole bearing no small resemblance to a multi-spiral shelly 

 as one of the Cerithiadce or Turritelladce. The ecthorceum 

 is discharged reluctantly from this form, and I have never 

 seen an example in which the whole had been run off. 

 So excessively subtile are the walls of the cnidce, that it 



CNIDA OF CORYNACTIS 



