1894,] The Classification of the Arthropoda. 223 
Order I—Xiphosura. 
Cephalothorax large, metastoma paired, telson elongate 
and spiniform, 
Sub Order [-ahitenelides.' 
Abdominal somites six, coalesced. 
Sub-Order [I—Hemiaspida. 
Abdominal somites more than six, free. 
Order II—Eurypterida. 
Cephalothorax small, abdomen large and elongate, twelve- 
jointed, the joints free, telson spatulate, metastoma unpaired. 
Susp-Ciass II—ARACHNIDA. 
Respiration by internal lungs or tracheæ, no compound eyes 
Entodermal Malpighian tubes present; Embryonic membrane 
(amnion) present in some. 
I regard the Scorpionida as the most primitive type of 
Arachnida existing to-day, and the Acarina as the farthest 
removed from the original stem. This position of the Scor- 
pions is shown by many facts of structure; and the pulmonate 
type of respiration—intermediate between the gills of the 
Gigantostraca and the trachee of the higher Arachnids—occur- 
ing in these forms is just what we should expect if the line of 
descent is, as here maintained, from branchiate forms. On the 
other hypothesis of a common origin of all “ Tracheates” from 
some Peripatoid form, we should have the strange spectacle of 
the most primitive of all Arachnids with the most differentia- 
ted respiratory system. 
In the Arachnida I recognize the following orders*—I, 
Scorpionida ; II, Thelyphonida ; III, Araneida, IV, Solpugida ;* 
V, Pseudoscorpii; VI, Phalangida; VIII, Acarina. 
It is interesting to note in this connection that Pocock, on 
morphological grounds points out (Ann. & Mag, Nat. Hist., VI, 
3 In this I follow the order of Pocock (Ann. & Mag. N. H., Jan., 1893). His 
“sub-classes’? Ctenophora, Lipoctena and super ‘ordinal divisions Chaulogastra, 
Mycetophora and Holostomata are hardly to be regarded as of phylogenetic value. 
*No group of Pepee will better ii study than this. I do not believe that the 
distinction between ‘ head” and the “ thorax” with its three distinct somites in- 
dicates any sacra aot the ik but that the conditions here existing are to 
be best explained upon the ground of homoplassy. The position of the anterior stig- 
mata in the first thoracic somite is of great interest, 
