37G ME. H. M. BERNAED OX THE 



which both could be deduced. Do the Arachnids throw any light upon the original nature of this 

 structure ? 



In the diagrammatic restoration of the abdominal limbs of Ga/eodes (PI. XXIX. fig. 11) I have 

 marked a row of structures which, from their positions with reference to the limbs, appear to form 

 a homologous series : — 



On the 3rd segment are the apertures of the coxal glands. Although these open in Galeodes on the 

 soft membrane, in the Pseudoscorpions they open (on the 5th segment) on the hard posterior faces of the 

 coxae. These may perhaps not really belong to the series*. On the next segment there is a pair of 

 stigmatic openings into tracheae. On the next two segments, judging from the circulatory system, we 

 are justified in assuming that there were once stigmata opening into tracheae. On the genital segment 

 the limbs have been folded together to guard the median genital aperture, but opening on the inner 

 faces of the limbs, in a position exactly corresponding with the tracheae, are a pair of glands [gg) which 

 appear to yield a secretion playing some part in the reproductive process ; they perhaps yield a sticky 

 substance to glue the eggs together. In Phrynus and Thelyphomis a pair of glutinous glands, capable 

 in the former of yielding irregular threads, is found in what appears to be the same spot (PI. XXIX. 

 fig. 13). Following these glands, there ai'c, on the three segments, tracheae, or their remains, opening 

 in positions exactly corresponding with the apertures of these glands. Lastly, in the Spiders, on the 

 same (4th) segment in which the aborted tracheal invagination occurs in Galeodes, and nearly on the 

 same spot, the 1st pair of median spiuuing-glands is found. 



If, then, these form a true series — and, with some qualifications to be mentioned later, I think they 

 do — we have some additional evidence as to the homology between tracheae and spinning-glands, 

 i. e. I believe with setiparous glands from which spinning-glands can best be deduced (see further, 

 pp. 323 and 383). 



The development of tracheal invaginations on the legs of Phalangids (Hansen), which may, however, 

 only mean the finding of new apertures on the part of long existing tubes, is certainly not opposed to 

 this theory. No known " dermal" glands occur in such positions on the legs of Arachnids, but spines 

 and setae with large setal pores are very common, and each one would form a place where the tracheal 

 tubes could without difficulty " rise to the surface.'" An examination of the way the seta is suspended 

 in the setal pore by a thin flexible fold of chitiu shows that this is the probable solution of the 

 occurrence of these leg-stigmata (see the figures of hairs on PI. XXX.). 



I shall have to return to the subject of the origin of trachea; in the next section. 



Histological (PI. XXX. figs. 17 ((, 17 h). — The large tracheal tubes of Galeodes show, 

 in section, the structure of their walls. We have the glassy layer of the cuticle lining 

 tlie whole tube. This is found in distended specimens forced up to form great blisters 

 projecting iuto the lumen of the tracheae. These blisters are full of coagulum. It is 

 hardly possible that this can be a normal adaptation in cases of extreme distension. It 

 is more probable that, in the death-agony, muscular contraction forces the blood through 

 the walls of the trachese so as to blister up the innermost hard layer. Following this 

 layer is the chitinogenous staining layer. This forms the thick hooj)s much in the 

 same way as it forms the lens (PI. XXXI. fig. 7). The lamina3 seem to swell and thus 

 to form ridges over which the hard layer passes. In the two figures given, both drawn 

 from sections under the microscope, the ridges are not at equal distances, and some 

 difference seems to exist between the thicknesses of the chitinogenous layer between 

 the ridges. Outside this latter layer comes the plate-like epithelium with the nuclei. 

 These are clear disc-like vesicles, often with only one very minute staining-point ; they 

 are thus not easy to demonstrate. 



* Oil this see note, p. 380. 



