SECT. XVI PERIPATUS AND THE TRACHEATA 287 



from the fact that it is accompanied by the development 

 of the Malpighian tubules, except in Peripatus, where 

 the nephridia are retained. The loss of the dermal 

 excretion necessitates the further development of 

 other excretory surfaces. The advantages of the 

 Malpighian tubules, or glandular caeca of the hind-gut, 

 over excretory organs in any other part of the body 

 have been already dwelt upon. This physiological 

 connection between tracheae and Malpighian vesicles 

 which lessens the improbability of their concurrent 

 development tivice^ cannot however be taken advantage 

 of in the special case under discussion, because the 

 tracheae are not supposed to have been dermal glands 

 but imbedded external gills. 



The early differentiation of the Arachnida from the 

 original Tracheatan-Annelids accounts for the high 

 specialisation of their tracheal gills. The same may 

 be said of the Myriapoda, while Peripatus has remained 

 in this respect, as in so many others, almost entirely 

 undifferentiated. 



In addition to these arguments we have to refer on 

 the one hand to those brought forward in this essay to 

 show that Limulus is a Crustacean, and on the other 

 hand to the discovery of rudimentary antennae in the 

 embryo of a spider,^ which removes the only difficulty 

 in the way of homologising the limbs of the Arach- 

 nida with those of the Antennata. 



It seems to us that we find evidence of the early 

 specialisation of the Arachnida, not only in the loss 

 of the antennae, in the form of the limbs and tracheae, 



^ Trochosa Singoriensis Laxm. See Zool. Anzeiger, May ii, 1891. 



