24 Transactions of the Society. 



found tliat there was not any difficulty in carrjdng it back as far 

 as the point where the tectum was supposed to commence, 

 but there it stopped, and that nothing would get it any farther, 

 whereas it ought to have passed equally easily into the supposed 

 space between the tectum and the dorsum of the cephalothorax 

 (or vertex as Nicolet calls it). It then struck me that amongst all 

 the very large number of specimens of Orihatidx which I had 

 examined I had not ever seen one where any dirt had got into this 

 space, although it gets into every other place where there is a 

 small hole or depression, and this would be a receptacle just fitted 

 for it ; but 1 thought that possibly a thin liquid, such as alcohol, 

 or even water, might run in where a hair or solid matter would 

 not pass. I accordingly tried, but could not get any to run under 

 the supposed tectum ; by these methods, but more especially by 

 careful and frequently repeated dissection of various species, I 

 at last became convinced that the tectum of Nicolet did not exist, 

 and that the appearance of it in Cepheus, &c., was an optical 

 delusion. In Orihata ^punctata, &c., the greatly enlarged and 

 horizontal lamellas somewhat simulate a tectum ; this form isNicoIet's 

 sub-parallel blades. 



How does the appearance of Cepheus, &c., arise ? It seems to me 

 that the explanation is as follows : — The lamellae are real and exist- 

 ing organs, easily seen by even the most superficial observer, and 

 quite easy to get away, by cutting or breaking, but instead of being 

 the upturned edges of a special, detached, horizontal, chitinous organ, 

 they are simply outfoldings of the cuticle of the cephalothorax 

 itself, just as the apodemata which serve as points of attachment for 

 so many of the muscles, are infoldings of the same cuticle ; in 

 this manner it is natural that the base of each lamella should be 

 thicker than its summit (or edge), which indeed does not include 

 the true cuticle at all, but only the chitinous secretion from it, and 

 the lamella, being a fold of the cuticle, does not spring sharply at 

 right-angles from the surface of the cephalothorax, but rises m a 

 curve, produced, so to speak, by the dragging up of the cuticle 

 from each side ; thus each lamella has a more or less triangular 

 transverse section, the sides being curved and giving considerable 

 extension to the base, particniarbj on the inner side. The dorsal 

 surface of the cephalothorax, from which these lamellae spring, is 

 convex, and the broad inner base of the lamella filling up the 

 depression caused by the lower part of the convexity, causes the 

 whole space within the lamellae to appear, and really to be, higher 

 in level than the other parts outside the lamella. Again, the ends 

 of the lamellae usually rise forming projections, sometimes very 

 short, but often of considerable length, which stand quite free. 

 At the point where the lamellae cease to be attached by their lower 

 edge?, and rise to form the free projections, ihe two lamellae are 



