TRACHEAL CONVERGENCE 149 



the contrary, are serially homologous with the 

 pulmonary sacs of the preceding somite, and 

 actually homologous with the second pair of 

 pulmonary sacs in Tetrapneumonous spiders. 1 



Purcell affirms that the arguments in favour of 

 the branchial origin of the lung-books of spiders, 

 advocated by Lankester in 1881, appear over- 

 whelming. An interesting analogy may there- 

 fore be drawn between the Pulmonate Arachnida 

 and the Pulmonate Mollusca, where a lung- 

 chamber has likewise been substituted for a 

 gill-chamber ; and just as in Arachnida, accord- 

 ing to Pocock (1893), there is reason to believe 

 that tracheal tubes have replaced lung-books at 

 least twice, namely, in the Dipneumones and in 

 the Pseudoscorpiones ; so in the Land Mollusca 

 a pulmonary chamber has twice replaced a gill- 

 chamber, namely, in the Land Operculates, which 

 belong to the order Prosobranchiata, and in the 

 inoperculate Pulmonata, which are related to the 

 Opisthobranchiata. 



The internal tracheae (of cutaneous origin) 

 are not homologous throughout the Arthropod 

 phylum — they are polyphyletic like the external 

 cutaneous branchiae of soft-bodied invertebrates. 



Now with regard to the trematic mode of 

 aquatic respiration, or breathing by means of 

 gill-clefts, the question naturally arises whether 



1 W. F. Purcell, "Development and Origin of the Respiratory 

 Organs in Araneas," Quart. Journ. Mia: Sc, vol. liv., 1909, pp. 

 1-1 10 ; contains full bibliography. 



K 2 



