ARTHROPODA 483 



G. ARTHROPODA (Jointed-limbed Animals). 



BY 



FRED. V. THEOBALD, M.A. 



BILATERALLY symmetrical segmented animals which are covered with a thick 

 cuticle that is frequently calcareous (Crustacea), but always thinner between the 

 segments ; they carry (primitively) a pair of jointed appendages on every segment. 1 

 The segments of the body are uniform in certain regions, but differ from those of 

 contiguous regions, so that it is easy to distinguish three parts (head, thorax and 

 abdomen), each composed of segments. The cephalic segments are always formed 

 into a uniform head, the segmentation being scarcely recognizable at either end ; 

 the thoracic segments may also fuse, or part or all of them may coalesce with the 

 head ; the abdomen, as a rule, retains its segmentation, but this may possibly also 

 be lost, in which case it is [sometimes] united to the cephalothorax. The structure 

 of the three regions depends mostly on the varying form and function of the 

 appendages : those on the head are primitively locomotive organs (and frequently 

 are still so in the early stages), but they become transformed into feelers and mouth- 

 parts (mandibles, maxillae) ; the limbs of the thorax, however, usually retain their 

 ambulatory functions, as frequently do those of the abdomen ; sometimes, however, 

 the abdominal limbs disappear, entirely or partly; in the latter case they are then 

 utilized for other purposes. 



In their organization the Arthropoda approach the segmented worms. 



The Arthropoda are generally divided into five groups (Crustacea? Protracheata, 

 Arachnoidea, Myriapoda? and Insecta or Hexapoda\ of which only the Arachnoidea 

 and the Hexapoda interest us here. 



A. ARACHNOIDEA (Spiders, Mites, etc.). 



The head and thorax are always united together ; the abdomen is 

 either segmented or without exterior segmentation, in which case it is 

 united with the cephalothorax. 4 The number of pairs of appendages 



1 [In most Arthropoda the skin is hardened by a deposit of chitin (Hexapoda, etc.). 

 F. V. T.] 



2 Parasitic or free-living Crustaceans may now and then invade man abnormally. Thus, 

 according to Betten, Caligtts curtus invade the cornea (Betten, R. A., " Par. Crust, as a 

 Foreign Body on the Cornea," Lancet, 1900, i, p. 1002; and Centralbl. f. Bakt. u. Par., 

 xxix, p. 506). According to Laboulbene, also Gammarus pulex (Laboulbene, A., " Obs. 

 d'accid. caus. par le G. fid. apport. avec 1'eau de boison dans 1'estomac d'un homme," Bull. 

 Acad. med., 1898, p. 21). 



3 R. Blanchard has compiled thirty-five cases in which Myriapoda have been observed in 

 the intestine as well as in the nose of human beings ("Sur le pseudopar. d. myriap. chez 

 1'homme," Arch, de Par., 1898, i, p. 452). E. Munoz Ramos reports an additional case 

 (ibid., p. 491). A few years ago a doctor in East Prussia sent me a rain worm out of a lady's 

 nose (cf. Hanan, A., " Wahrsch. Pseudoparas. v. Schweiss fliegenlarv. u. angebl. Paras. 

 v. Regenwurmern b. ein Hysterisch," Arch, de Far., 1899, ii, p. 23). 



4 [This is only so in the Acarina or mites, not in the Araneida or spiders. F. V. T.] 



