COMPARATIVE MORPHOLOGY OF THE GALEODID.E. 



373 



c.ch. = cardiac chamber. 



stig. = stigmata. 



v.p. = veno-pericardial muscles, 

 st.sc. = stigmatic scar. 



lu the last column of this table I have placed the Pseudoscorpions inasmuch as they appear to stand 

 alone. Like Scorpio, they have lost all traces, external and internal, of the former existence of 

 cephalothoracic stigmata. But the compression of the segments to which the loss is attributable is quite 

 different in the two cases. In Scorpio, the compression was from behind forward, owing to the forward 

 thrust of the genital aperture ; in the Pseudoscorpions, from before backward, the four posterior limbs 

 having been squeezed back by the enormous coxfe of the pedipalps. In the Pseudoscorpions we have the 

 ram's-horn organs opening under the genital opercula, which I suggest (lo) may have been a primitive 

 form of tracheal invagination. In the two following segments are two pairs of highly specialized 

 stigmata, while on the seven following segments occurs a remarkable series of scars, a row on each side 

 segmentally repeating the functional stigmata, the last pair being found on each side of the anal papilla. 

 Internally, the heart has degenerated, but there are seven functional blood-passages running up between 

 the alimentary diverticula along the dorso- ventral muscles (PI. XXXIV. fig. 3). 



We have then traces, either internal or external, of 11 pairs of stigmata in Thtlyphonus, 10 in the 

 Pseudoscorpions, 9 in Galeodes, 8 in Scorpio, ranging thus from the 4th cephalothoracic to the 10th 

 abdominal segment, i. e., in all in 14 segments. 



We might, indeed, have inferred a priori from the traces of limbs found on all the segments of 

 Galeodes and (?) on the last segment in Scorpio {cf. PI. XXIX. fig. 12) that these limbs at one time 

 had some primitive form of trachea (or even more primitive gland from which the trachea may be 

 deduced) associated with them, since, in all the tracheate Arthropods, stigmata are invariably 

 associated with limbs. 



The only segments, then, on which hitherto we have found no traces of the former occurrence of 

 stigmata are the three most anterior. The presence of limbs, again, suggests that there were at one 

 time primitive trachese or the structures out of which tracheae developed on these limbs also. Certain 

 Acaridse are reported to have trachete associated with the 1st pair of limbs, but there is no record of 

 trachese on the 2nd and 3rd segments. It will be remembered that the fusion and compression of these 

 three anterior segments was, according to our theory of the origin of the Arachnida, the first special- 

 ization of the Class, the first step in adaptation to a method of feeding which led to the differentiation 

 of the phylum from their Annelidan ancestors. This compression may therefore account for the 

 disappearance of all traces of the stigmata. (On the suggested homology of the coxal glands on the 3rd 

 segment with the trachete, see p. 380 and footnote.) 



