446 



ARTHROPODA. 



The respiratory organs already alluded to (p. 443) have their 

 spiracles, always few in number, on the anterior ventral part of 

 the abdomen and, it is stated, sometimes on the cephalothorax. 

 The internal organs are the lungs and the tracheae. A lung is a 

 rounded sac just inside the spiracle and consists of numerous leaves 

 on the anterior wall of the lung sac. Each leaf is covered on each 

 side by a thin layer of chitin and contains a blood space in its in- 

 terior, while between the leaves are flattened spaces into which the 

 air enters (fig. 456). The tracheae, on the 

 other hand, are branched tubes arising from 

 the abdominal spiracles and penetrating 

 the abdomen (fig. 460). These are lined 

 with chitin, and to strengthen them with- 

 out undue thickness this lining is thrown 

 into folds, usually arranged in a spiral. 

 In the scorpions and tetrapneumonous 

 Araneina only lungs occur. In other 

 spiders one pair of lungs is replaced by 

 tracheae, while in most other arachnids only 

 tracheae occur. (The smaller mites and 

 FIG. 460. Beginning of paired parasites lack specialized respiratory or- 



trachese of Anyphozna uccen- , . , , ,, . 



taatn. (After Bertkau.) st, gans and circulatory organs as well.) 

 These facts, aside from embryological con- 



ditions, show that lungs and tracheae are morphologically equiva- 

 lent. The localization of respiration in the abdomen has resulted 

 in having the heart in the same region. It is noticeable that, as 

 the tracheae are developed, the circulatory vessels are reduced. In 

 the scorpions, which have only lungs, the circulation is most 

 nearly complete. 



In development the arachnidan tracheae arise from the abdominal 

 appendages, as do the lungs. (In the Solpugidse and some mites cepha- 

 lothoracic tracheae occur, but nothing is known of their development.) 

 This fact shows that they are entirely different in origin from the tracheae 

 of insects, while numberless details show that these structures are only to 

 be compared with the gills of Limulus. 



The gonads (only the Tardigrades are hermaphroditic) are 

 abdominal in position and open by paired ducts (sometimes with a 

 single mouth) on the first abdominal somite. In most cases the 

 animals are oviparous, but the scorpions and many mites bear liv- 

 ing young. In many instances the mothers care for their eggs and 

 young, the scorpions carrying their families on their bodies. Only 

 rarely is there a metamorphosis, and then in the aberrant forms 



