242 TRILOBITA 



some forms of Apus the second pair of antennae may be rudi- 

 mentary or even absent. 



There are still other features which characterise the Trilo- 

 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. 1 He ^points out that some of the 

 earliest Trilobites, such as Holmia kjerulfi (Fig. 148), possess 

 nearly the same number of segments as Nebalia (Fig. 76, p. Ill), 

 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 2 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. H. Irish Acad. xxiv., 1903, p. 332, and Quart. Journ. Micr. 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. 1., 1894, p. 433 ; Bernard, ibid. vol. 1. p. 432. 



