604 MORPHOLOGICAL METHOD AND RECENT PROGRESS IN ZOOLOGY. 



As to the tracheal system, now adequately recognized by the 

 upholders of the arachnid theory, the presumed origin of trachea3 

 from lung })ooks, the probability that the ram's-horn organ of the 

 Chernetid^ ma}^ be tracheal, the presence of tracheae in a simple form 

 in the Acari, and, by way of an anomaly, in a highly organized form 

 on the tibise of the walking legs of the iiarvest men (Phalangidt^), are 

 all features to be borne in mind. While I am prepared to admit that 

 this wide structiu'al range and varied distril)ution of the trachese less- 

 ens their importance as a criterion of affinity, I can not accept as 

 conclusive the evidence for the assumed homology between lung l^ooks 

 and gills. And here it may be remarked that a series of paired 

 abdominal vesicles, recently found in the remarkable arachnid luBjien ia, 

 invaginate as a rule, but in one example everted, seized upon in 

 defense of this homology, have not been so regarded l)y those most 

 competent to judge. 



There remains the entosternite, an organ upon wliich nuich empha- 

 sis has been placed. Not onl}" does a similar organ exist, apart from 

 an endophragmal system, in Ajjus, Lhjelops^ some Ostracods and Deca- 

 pods: but, regarding the question of its histology, it may l)e pointed 

 out that from all that is at present known the structural differences 

 between these several entosternites do not exceed those between the 

 cartilages of the Sepia body. And when it is found that the figures 

 and descriptions of the entosternite of Mijiiale (" My gale sp. ," " M^^gal- 

 omorphous Spider," <7^^c?^.) have been thrice presented upside down, 

 the reliability of this portion of the argument is lessened, to saj^ the 

 least. 



Recent observation has sought to clench the homology of the four 

 posterior pairs of limbs of the King crab and Scorpion, by appeal to a 

 furrow on the fourth segment in the former, believed to denote an 

 original division into two; but I hesitate to accept this until myologi- 

 cal proof has been sought. 



Returning, amidst so much that is problematic, to the sure ground 

 of paleontology, 1 wish to point out that when all is considered in 

 favor of the arachnid theory there still remains another way of inter- 

 preting the facts. 



In both Linmlus and the Scorpion the first six of the eighteen seg- 

 ments are well known to be fused into a prosoma l)earing the limbs, 

 but while in the Scorpion the remaining twelve are free, in Linuilus 

 they are united into a compact opisthosomal mass. In dealing with 

 the living arthropods, there is no chiiract(M- determinative of position 

 in the scale of this or that series more trustworthy than the antero- 

 posterior fusion of segments. It has been called the process of 

 " cephalisation,'' and tlu; degree of its l)ack\v'ard extension fui'iiishes 

 the most reliable standard of higlmcss or lowiiess in a given assem- 



