INTRODUCTION. 
XXVll 
membrane of exeessive tenuity, and one or two scattered 
cnidce, across the bright interval. On another occasion, in 
the case of a cinclis at the edge of tlie parapet — a position 
singularly favourable for observation — I saw that this 
subtle film was gradually pushed out until it assumed the 
form of a hemisplierical 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 farther 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 
deorganized 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 contractions 
of the animals, 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. 
That the cinclides are the special orifices through 
which those missile weapons, the acontia, are shot and 
recovered, rests not merely on the probability that arises 
from the coexistence of the two series of facts I have 
above recorded, but upon actual observation. In a rather 
large S. dianthus, somewhat distended, placed in a glass 
vessel between my eye and the sun, I saw, with great dis- 
tinctness, by the aid of a pocket-lens, many acontia 
protruded from the cinclides, and many more of the latter 
widely open. The acontia, in some cases, did not so 
accurately fill the orifice but that a line of bright light (or 
of darkness, according as the sun was exactly opposite 
or not) was seen, partially bordering the issue of the 
thread, while the thickened rim of the cinclis surrounded all. 
The appearance of the orifices whence the acontia 
issued was that of a tubercle or wart, and the same appear- 
ance I have repeatedly marked in examples observed on 
the stage of the microscope; namely, that of a perforate 
pimple, or short columnar tube. This was clearly manifest, 
when the animal, slowly swaying to and fro, brought the 
sides of the cinclis into partial perspective. 
On another occasion I witnessed the actual issue of the 
acontia from the cinclides. I was watching, under a low 
