XXVI INTRODUCTION. 



eye, supposing these to be reversed ; the convexity being 

 inwards. Each is an oval depression, with a transverse 

 slit across the middle. When closed, this slit may some- 

 times be discerned merely as a dark line (Plate XI. fig. 5, m), 

 the optical expression of the contact of the two edges ; but, 

 when slightly opened (/), a brilliant line of light allows the 

 passage of the rays from the lamp to the beholder. From 

 this condition the lids may separate in various degrees, 

 until they are retracted to the margin of the oval pit, and 

 the whole orifice is open (k). 



The dimensions of the cinclides vary not only with the 

 species, and probably also with the size of the individual, 

 but with the state of the muscular contraction of the integu- 

 ments, and, as I think, with the pleasure of the animal. 

 In a small specimen of 8. dianthus, I found the width of 

 a cinch's, measured transversely, 2 75th of an inch ; but that 

 of another, in the same animal, was more than twice as 

 great, viz. ribth of an inch. This was on the thickened 

 marginal ring, or parapet, which in this species surrounds 

 the tentacles, where the cinclides are larger than elsewhere. 

 Watching a specimen of 8. nivea under the microscope, 

 I saw a cinch's begin to open, and gradually expand till it 

 was almost circular in outline, and %} 5th of an inch in 

 diameter. I slightly touched the animal, and it in an 

 instant enlarged the aperture to --froth of an inch. In a 

 specimen of 8. bellis, less than half grown, I found the 

 cinclides numerous, and sufficiently easy of detection, but 

 rather less defined than in dianthus or nivea. They occurred 

 at about every fourth intersept, three intersepts being blind 

 for each perforate one, and about three or four in linear 

 series, but not quite regularly, in either of these respects. 

 In this case they were about gVth of an inch in transverse 

 diameter, a large size, — and I measured one which was 

 even ^th of an inch. By bringing the animal before the 

 window, I could discern the light through the tiny orifices 

 with my naked eye. 



From several good observations, and especially from 

 one on a cinch's, widely opened, that happened to be close 

 to the edge of the parapet of a dianthus, I perceived that 

 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 



