AlfATOMT OF SPH^EOTHERIUM. 167 



But beyond the bare mention of the fact in the work above 

 quoted, I have not found a description of their trachese in any 

 other author ; and the differences in the fcrachege are generally 

 considered to rank among the leading distinctions between the 

 Diplopoda and Chilopoda. 



It seems that the tracheae oi Sphtsrothermm are a transition from 

 those of the Jidus type to those of the Scolopendr a type. I do 

 not, however, infer that any genetic relationship exists between 

 Sphcerotherium and the Chilopoda. The whole anatomy of the 

 former points to its beiag a highly specialized Diplopod, in which, 

 in accordance with its high specialization, the tracheal system has 

 become more fully developed than in the other members of the 

 group to which it belongs ; and the development has been in the 

 same direction as in the Chilopoda and Insecta. Tlie resemblance 

 in fact is due to homoplasy, not to homology. 



The trachese of Sphcerotlierium may be derived from those of 

 Julus and Peripatus ; and the tracheal sacs are, no doubt, homo- 

 logous with the tracheal sacs in both these forms. The presence 

 of the tracheal sac is a primitive character which is found only 

 in the Prototracheata and Diplopoda. In SphcBrotherium the 

 primitive character of the sacs is altered in so far that the 

 tracheal stems are no longer given off as a tuft from the surface 

 of the tracheal sac, but two primary stems are given off, one 

 from the dorsal part of the sac lying nearest to the median line, 

 the other from the extreme lateral prolongation of the sac, with 

 which the tracheal stem appears to be continuous. The primary 

 stems soon branch ; both they and their branches have well- 

 developed spiral filaments, and their ultimate ramifications extend 

 through all the tissues of the body. 



It may be ohserved that in SpTicerotherimn the openings of the 

 tracheal sacs on the surface of the body are clearly the stigmata, 

 and not the openings of the trachese into the tracheal sacs. This 

 is worth mentioning, because a writer on Peripattis has denied 

 that the sac-mouths are the true stigmata, and has confined that 

 name to the openings of the unbranched trachese into the sacs 

 (GrafEron, " Beitrage zur Anat. und Histologie von 'Peripatus^'' 

 Zool. Beitrage von A. Schneider). He regards the latter simply 

 as invaginations of the skin ; but the trachese themselves are 

 nothing more than invaginations of the skin modified for respi- 

 ratory purposes ; and it is better to regard the tracheal sacs as 

 an integral part of the tracheal system, and the sac-mouths as the 

 true stigmata. 



