406 EVENINGS AT THE MICROSCOPE 



the passage is not absolutely open, at least in ordinary, but 

 that an excessively thin film lies across it. By delicate 

 focusing I have detected repeatedly, in different degrees 

 of expansion, and even at the widest, the granulations of 

 a membrane of excessive tenuity, and one or two scattered 

 cnidce, across the bright interval. On another occasion, in 

 the case of a cinclis at the edge of the* parapet, a position 

 singularly favorable for observation, I saw that this subtle 

 film was gradually pushed out until it assumed the form 

 of a hemispherical bladder, in which state it remained as 

 long as I looked at it. At the same time the outline of 

 the cinclis itself was sharp and clear, when brought into 

 focus further in. The film, whatever it be, is superficial, 

 and does not appear to be a portion of the integument 

 proper. I take it to be a film of mucus (composed of de- 

 organized epithelial cells), which is constantly in process 

 of being sloughed from all the superficial tissues in this 

 tribe of animals, and which continues tenaciously to invest 

 their bodies, until, corrugated by the successive contrac- 

 tions of the animal, it is washed away by the motions of 

 the waves. As, however, one film is no sooner removed 

 than another commences to form, one would always expect 

 external pores so minute as these to be veiled by a mucus- 

 film in seasons of rest. 



"The pressure of this film is sufficient evidence that the 

 cinclides are not excretory orifices for the outflow of the 

 respired water, in the manner of the discharging siphon 

 in the Bivalve MOLLUSCA at least that no current con- 

 stantly, or even ordinarily, passes through tnem. I have 

 watched them continuously for periods sufficient to detect 

 such discharge if it were periodic. On one occasion (viz., 

 that in which the film was protruded like a blown bladder) 



