‘ey 
Si ha 
1903.] PACKARD—CLASSIFICATION OF ARTHROPODA. 143 
poda might be retained. The fundamental characters are the pos- 
session of jointed or polymerous appendages, and the great reduc- 
tion or entire absence of a ccelomic cavity. Besides this the ante- 
rior body-segments are grouped into a head, while the trunk-seg- 
ments may be either separate and homonomous or differentiated into 
a thoracic and abdominal region. But it has been pointed out by 
Kingsley and also by Laurie that the possession of jointed legs in 
the different classes may be due to convergence, or to homoplasy. 
Kingsley and others have shown that the gills and trachez are 
adaptive characters, and that the retention of the groups Branchiata 
and Tracheata is not warranted. Gills and trachez are adaptive 
features. We have in the phylum Paleopoda the classes of bran. 
chiate Trilobita and branchiate Merostomes, while from the latter 
appear to have evolved the terrestrial tracheate Arachnida. The 
mode of respiration affords fair class characters, but not phylum 
characters. 
HISTORY OF OPINION AS TO THE POLYPHYLETIC NATURE OF 
ARTHROPODA. 
As early as 1869 the present writer’ rejected Miiller’s (1864) and: 
Haeckel’s view (1866) that the insects and other tracheates had 
descended from the zoéa of the Crustacea, and claimed their ances- 
try from the Annulata. Kennel?* in 1891 stated his view that the 
Crustacea arose by an independent line of descent from that of the 
Annelida, the two groups having diverged from a Preannelidan 
ancestor, his Protrochosphera, from which the Mollusca also sprang. 
The tracheate classes he traces back to the Peripatiformes, from 
1 My views were stated in an article, entitled « The Ancestry of Insects,” in 
the American Naturalist, iii, p. 45, March, 1869. In commenting on Haeckel’s 
view that the ancestor of insects, spiders, and myriopods was a zoéa-like form, a 
view previously expressed by Fritz Miiller, and also held by Dohrn, I rejected 
this theory and suggested that the ancestor of insects and other tracheates “ must 
have been worm-like and aquatic.” A little later I referred the ancestry of both 
the insects and crustacea, “independently of each other, to the worms (Annu- 
lata)” (American Naturalist, iv, p. 756, February, 1871). 
2« Die Verwandtschaftverhaltnisse der Arthropoden (Schriften Naturf. 
Gesells. Dorpat, vi, 1891). Kennel’s view that the Nauplius form originated 
from the Rotatoria was earlier expressed by the writer, as follows: “The Nau- 
plius form of the embryo or larva of all Crustacea also points back to the worms 
as their ancestors, the divergence having perhaps originated in the Rotatoria”’ 
(American Naturalist, v, p. 52, March, 1871). 
