VI 



ARACHNOIDEA OUTER ORGANISATION 



517 



the Arachnoidea branched off from the common Tracheatan stem at a time when the 

 oral appendages were still much jointed, and elongated like legs. 



In connection with the assumed complete absence of the Antennatan feelers in the 

 Arachnoidea or Chelicerota, it is a striking fact that no rudiments which can be 

 proved to be those of antennae appear, as far as we know, at any stage of development, 

 even temporarily. 1 And yet we must assume, keeping Peripatus in mind, that the 

 ancestors of the Tracheata possessed well-developed antennae. 



In recent times the near relationship of the Arachnoidea, and especially of the 

 Scorpionidce, with the fossil Gigantostraca and the Xiphosura has been zealously 

 maintained. It cannot be denied that the 6 pairs of limbs of the Scorpion show- 

 greater agreement with the 6 pairs of limbs of the cephalo-thorax of Limulus than 

 with the limbs of the Antennata. There are, however, other serious objections to the 

 assumption of a nearer relationship between the Arachnoidea 011 the one hand and 

 the Xiphosura and Gigantostraca on the other (see p. 541). 



Rudiments of Abdominal Limbs in the Arachnoidea. 



1. In various Arachnoidea rudiments of abdominal! limbs appear temporarily 

 during embryonic development ; 6 pairs on the 6 anterior abdominal segments in the 

 Scorpionidce (Fig. 379, p. 538), 4 pairs in the 



Ghernetidce and 4 pairs in the Araneidce (Fig. 

 380, p. 539). Such rudiments of abdominal 

 limbs will probably also be found in the 

 embryonic stages of other Arachnoidea, whose 

 development has been hitherto not sufficiently 

 investigated. 



2. The Scorpionidce possess in the adult 

 condition on each side of the second abdo- 

 minal segment ventrally a comb-like organ 

 (Fig. 362, k), whose function is not yet fully 

 known. These two "combs" are said to 

 come from embryonic rudiments of the limbs 

 of the second abdominal segment. 



3. It is in the highest degree probable 

 that the spinning mammillae of the Araneidce, 

 which, 4 or 6 in number, rise on the hind 

 body, represent rudimentary abdominal limbs. 

 This is supported by the following facts : (a) 

 they are for the most part jointed ; (b) they, 

 as recently observed, develop from the em- 

 bryonic rudiments of abdominal limbs ; (c) the 

 fact that they are spinning mammillae, i.e. 

 that the spinning glands open 011 them. 

 These facts gather weight when we remember 

 the coxal and spinning glands of the Pro- 

 tracheata and Antennata, and particularly the 

 slime papillae of Peripatus and the spinnerets 

 of Scolopendrella. The assumption that the 

 spinning mammillae of the CypliopJiihcdmid 



genus, Gibbocellum, also represent rudimentary limbs, is open to the objection that 

 the two pair of mammillae lie in one and the same, i.e. the 2d abdominal segment. 



FIG. 362. Buthus occitanus (Regne 

 animal). Cephalo-thorax, pre- abdomen 

 (pra), and the first segments of the post- 

 abdomen ( pa), from the ventral side. Limbs 

 (1-6) not fully drawn. 1, Chelicerse; 2, 

 pedipalps (jaw-feelers, chelate-feelers) ; g, 

 genital aperture ; s, stigmata ; k, combs. 



1 See note on p. 516. 



