68 



NA TURE 



[November i6, 1893 



THE STIGMATA OF THE ARACHNIDA, AS A 

 CLUE TO THEIR ANCESTRY. 



T F on a diagram of an Arachnidan body we mark every segment 

 -*■ on which stigmata are said to occur, the result is somewhat 

 remarkable. 



The subjoined figure is such a diagram, and stigmata, functional 

 or vestigial, are known on all the segments except the second 

 and third. On the left hand of the diagram I have recorded the 

 form which the tracheal invaginations from each pair of stigmata 

 assume, i.e. whether they are tubular or laminate ; and on the 

 ri|^ht hand I have lioted the genera in which the stigmata occur, 

 with a straight underline if their trachere are tubular, and a wavy 

 underline if their trachere are laminate. The marks which re- 

 present the stigmata are not intended to denote their real 

 positions on the segments, but only to indicate their presence, 

 although there is reason for thinking that they may well repre- 

 sent their primitive positions. 



On the first segment we have stigmata recorded on the dorsal 

 surface at the bases of the mandibles in some Acaridje. This 

 remarkable position may be in in some way connected with 

 the formation and dorsal arrangement of the " cephalic lobes," ^ 

 which accompanies the translocation of the mandibles from a 

 post oral to a pre-oral position. All the other tracheae of the 

 Arachnida are associated with limbs, the Arachnida agreeing in 



I 7ui(iUr 



H 



M 



TV Tuiu^Uy 

 it 7:Ualiy 



TTii.UtT -J 

 a T.Vu.ls.y- 1 



''' Lim.Cn iic 



/3 

 /^ 

 /S 



'Ji. 



fie^nlA. I 



' L^m^i^ny^ 





sent the closed stigmata of vanished tracheae, which trachea, 

 if we may judge from the ram's-horn organs and the functiona- 

 trachere on the second and third abdominal segments, were all 

 most certainly tubular. 



From these facts we are justified in concluding (i) that the 

 tracheal invaginations of the ancestor of the Arachnids were 

 strictly segmental, and (2) that these invaginations were of some 

 simple tubular type, from which the laminate forms could be 

 easily developed. The ram's-horn organs of the Chernetid?e, with 

 the simple air chambers in their epithelial cells as recently 

 de-cribed {I.e. supra), might well represent such primitive in- 

 vaginations. 



An impartial review of the facts must convince everyone that 

 the laminate form is the more specialised. The tubular form i& 

 by far the most widely distributed, being not only universal 

 among the Hexapoda and Peripatida:, but far more common 

 among Arachnids than the laminate. Tubular trachese occur 

 universally in the Acaridre, which have some claim to be con- 

 sidered as fixed larval forms {JoiiryLinn. Sac. Zool. vol. xxiv. 

 p. 279) ; in the Phalangida?, which a^e so specialised that they 

 must have br.inched off very early from the main Arachnidan 

 stem ; in the Chernetidse, which again are very difficult to class, 

 and may claim to be an independent group ; and lastly in the 

 Galeodida;, which in many respects are the most primitive of all 

 Arachnids. 



The Araneidae, with their highly specialised 

 abdomen, possess both laminate and tubular 

 forms, while the Scorpionidae and related 

 Pedipalpi are the only Arachnids, and indeed/ 

 the only Arthropods (except a few Myriapods), 

 which have exclusively laminate tracheae.. 

 The laminate trachese of the Myriapods 

 cannot possibly be deduced from the laminate 

 tracheae of the Arachnids, but both forms of 

 laminate tracheae can be deduced from a 

 primitive tubular organ. 



Returning to the diagram, the facts therein, 

 epitomised presuppose the existence in the 

 ancestor of the group of simple limbs on 





<ds. 



— ~~~ / D / ^^^''y segment. In addition to the six 



(^^^^^jj^^A.^ ^e±^£^:!^ ^^^t^X^"*" '[^^^^!^trz - ^^S^^Al pairs of limbs on the first six segments, 









his point with the Hexapoda and Myriapoda, but differing from 

 the Peripatidae. 



On the sixth segment we have stigmata both in Acaridoe and 

 Phalangidas behind the last pair of legs. This position may 

 mean either on the last cephalothoracic or on the first abdominal 

 segment. I do not wish to lay special stress on this one case ; 

 but, considering the general absence of stigmata on the abdomen 

 in Acaridx, owing to the rudimentary condition of this region of 

 the body, I think there is something to be said for the position in- 

 dicated in the diagram. The same argument appliesto the Phalan- 

 gidae, in which animals also the abdomen is but feebly developed. 



On the first abdominal segment (7) we have the remarkable 

 ram's-horn organs which I have elsewhere- suggested may be a 

 primitive form of tracheal invagination, from which either 

 laminate or tubular trachese may be very easily deduced. 



On the following two abdominal segments (8 and 9) we have 

 tubular and laminate tracheae equally distributed, all those 

 hitherto described having been tubular. 



On the tenth segment we have functional laminate trachere 

 in Scorpio, tubular tracheae descending to a medium (? closed) 

 stigma in Galeodes, and vestigial stigmatic scars in the Cher- 

 netidae. These scars are repeated segmentally on all the 

 remaining abdominal segments in the Chernetidac, and repre- 



' The ' Head ' of Galeodes,' 

 ' Notes on ihe Chernetidcc.' 



&c. Zool. Anz. No. 426, 1893. 

 Linnean Society's Joiirn, (in press). 



clear traces of limbs are well known either 

 in a persistent or a vestigial disappearing form 

 on the first six abdominal segments. Further, 

 we can conclude from the stigmatic scars- 

 on the Chernetidae, that there must originally 

 have been limbs on all the remaining segments- 

 of the abdomen. The character of these 

 limbs may perhaps be gathered from those of 

 the nymphs of many Acaridac, which probably 

 represent the earliest developmental stages 

 in the Arachnida now traceable, and perhaps, 

 also from the legs of the Silurian Scorpion, 

 PalfTophoiriis niincuis. 



We can thus trace the Arachnids back to a 

 segmented ancestor with a pair of limbs and a pair of tracheae 

 on every segment— /.^. to an ancestral form resembling those 

 of the Myriapoda and Hexapoda, but differing from these in 

 having the rows of stigmata ventral instead of lateral, apart, of 

 course, from the profound difference in the specialisation of the 

 oral appendages. 



Among the later modifications of this primitive Arachnid 

 we have the differentiation of the body into the cephalothorax 

 and the abdomen. The former consists of the first six segments 

 which have retained their limbs. The segments of the latter 

 have lost their limbs almost completely, rudiments, however, 

 persisting as genital opercula, pectines, and spinning mammillae. 

 In the former the great development of limbs and musculature 

 and the fusing of the segments together has led to a general 

 suppression of the tracheal invaginations. One pair, however, 

 persists in the Galeodidae, which, ivilh the exception of 

 Schizonotiis, are alone among Arachnids in retaining the 

 original jointing of the posterior cephalothoracic segments, and 

 in itie Acaridae and Phalangidae, in both of which the abdominal 

 region is not fully developed. 



The second and third segments appear to be the only ones on 

 which hitherto no traces of stigmata have been found. This 

 may be due to the fact that in the formation of the mouth parts 

 the first three segments early fused together. 



The general disappearance of tracheae from the cephalothorax 



NO. 1255, VOL. 49] 



