114 ARACHNIDA. 



organisation than the primitive forms, especially as certain systems 

 of organs (circulatory and respiratory systems) may partly, or 

 wholly, degenerate. 



The abdominal limb-rudiments are of peculiar importance in the 

 comparison of the Arachnida with other Arthropods. Their number 

 in the Scorpiones, as in Limulus, is six. [? cf. Brauer, Kishinodyb.] 

 It is possible that in the Araneae, also, the same number of 

 abdominal appendages was originally present (p. 51). The Arach- 

 nida, like the Insecta, were derived from forms provided with a 

 larger number of limbs. The first pair [second, pp. 10, 25, 57], is 

 related to the genital aperture, while the following pairs show on 

 their posterior surface the invaginations which give rise to the lungs. 

 The lungs of the Arachnida may therefore be homologised with 

 some probability with the gills of the Xiphosura (Vol. ii., p. 358, and 

 Vol. iii., p. 77). This implies an origin for the Arachnidan tubular 

 tracheae different from that in other "Tracheata" (Peripatus, Myrio- 

 poda, Insecta), for there can be no doubt that the tracheae in the 

 Arachnida are in the closest connection with the lungs.* Although 

 the tracheae in a few Arachnids, e.g., the Solifugae, the Opiliones, 

 and some Pseudoscorpiones and Acarina, seem to resemble each other 

 greatly in structure, they must, in the one case, be derived from 

 lungs or gills, and, in the other cases, from simple integumental 

 depressions. Their later similarity of structure must be regarded 

 as a phenomenon of convergence.! 



The presence of the stigmata in the abdomen only is in accord- 

 ance with the view of the origin of the respiratory organs here 

 adopted, but an exception occurs in the first pair of stigmata of the 

 Solifugae which lies on the first "thoracic," or, rather, fourth 

 cephalo-thoracic, segment. This must for the present be regarded 

 as a secondary acquisition, and we may similarly try to explain the 

 fact that, in the Acarina, stigmata occur in the cephalo-thorax at 

 various points, often very far forward, in the cheliceral region. 

 Similar displacements of the stigmata are also known to occur in 

 Scolopendrella, where they also appear in the head in an unusual 



manner. 



* [See Simmons and Purcell (App. to Lit. on Araneae, Nos. VII., VIII.) 

 and footnotes, p. 78.— En.] 



t [Tubular tracheae are not restricted to these four groups, but are also iouiirt 

 in many Araneae associated with the lungs ; only the Scorpiones and the Pedi- 

 palpi have lungs alone. This has led Bernard (App. to Lit. on Arachnida in 

 gen No. III.) and Jaworowski (App. to Lit. on Araneae, No. II.) to the 

 conclusion that the lung-books are not primitive structures giving rise to 

 the trachea, but rather that both the lung-books and trachea are to be derived 

 from simple sac-tracheae. — Ed.] 



