INTRODUCTION. XXVU 



membrane of excessive tenuity, and one or two scattered 

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

 the case of a cinch's at the edge of the parapet — a position 

 singularly favourable 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 cinch's 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 

 Reorganized 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 cinch's 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 



