242 TRILOBITA CHAP. 
some forms of Apus the second pair of antennae may be rudi- 
mentary or even absent. 
There are still other features which characterise the Tnlo- 
bita: thus the eyes are borne on free cheeks, and differ in 
structure from those of Phyllopods. The broad pygidium formed 
of fused segments and without terminal fulcra is quite unlike 
the slender-jointed abdomen of Apus and Branchipus ; and whilst 
in the Trilobites all the segments bear appendages, in the 
Phyllopods some, at any rate, of the posterior segments are 
devoid of appendages. The distinct division of the body into 
an axial and pleural region is not seen in Phyllopods, and is 
probably a character of some importance, since it occurs in the 
great majority of Trilobites, including all the early forms. 
The existence of some relationship between the Trilobita and 
the Leptostraca (Phyllocarida) has been maintained by Pro- 
fessor G. H. Carpenter.’ He points out that some of the 
earliest Trilobites, such as Holmia-kjerulfi (Fig. 148), possess 
nearly the same number of segments as Vebalia (Fig. 76, p. 111), 
and that in the latter genus the cephalic appendages, especially 
the mandibles and maxillae, are less specialised than in Apus, 
and consequently differ less from those of Trilobites than do the 
appendages of the Apodidae. Further, in another genus of the 
Leptostraca, Paranebalia, the biramous thoracic legs, in which 
both endopodite and exopodite are elongate, approach those of 
Trilobites more nearly than do the thoracic legs of Apus. 
The view” that some connexion may exist between the 
Isopoda and the Trilobita seems to have been based on the 
similar dorso-ventral flattening of the body, its division into 
three regions—head, thorax, and abdomen—and the presence of 
sessile eyes. Beyond this it is difficult to find any resemblance ; 
whilst the differences, such as the variable number of thoracic 
segments and their biramous appendages in Trilobites, are 
important. 
At present, then, we can only conclude that the Trilobita 
are more primitive than any other Crustacea, and that their 
resemblance to some of the Phyllopoda is sufficient to make 
1 Proc. R. Irish Acad. xxiv., 1903, p. 332, and Quart. Journ. Mier. Sci. xlix., 
1906, p. 469. 
2 This has received some support from H. Milne Edwards, Ann. Sci. Nat. Zool. 
(6), xii., 1881, p. 33; H. Woodward, Quart. Journ. Geol. Soc. xxvi., 1870, p. 487, 
and vol. ]., 1894, p. 483 ; Bernard, dbid. vol. 1. p. 482. 
