622 ARTHROPODA SUB-KINGDOM vir 



of abdominal segments in some genera is also to be considered as a primitive 

 character expressive of an annelidan style of growth. Genera, like As<i./>liu.< } 

 Phacops, etc., having a constant number of thoracic segments accompanied by 

 other characters of a high order, undoubtedly represent the normal Trilobite 

 type. 



These analyses and correlations clearly show that there are characters 

 appearing in the adults of higher and later genera, which successively make 

 their appearance in the protaspis stage, sometimes to the exclusion or modifica- 

 tion of structures present in the more primitive larvae. Thus the larvae of 

 Dalmanites or Proetus, with their prominent eyes and glabella distinctly 

 terminated and rounded in front, have characters which do not appear in the 

 larval stages of ancient genera, but which may come in their adult stages. 

 Evidently such modifications have been acquired by the action of the law of 

 earlier inheritance or tachygenesis. 



Position in the Zoological System. Since Trilobites have been made the sub- 

 ject of special study, they have been commonly classed with the Crustacea, and 

 placed near the Phyllopods by most observers. Quite a number of naturalists, 

 however, still divorce the Trilobites and Limuloids from the Crustacea, and 

 ally them with the Arachnids. Leaving aside the question of the homologies 

 of Limulus, it is a fact that Trilobites show the clearest evidence of primitive 

 Crustacean affinities, in their protonauplius larval form, their hypostoma and 

 metastoma, the five pairs of cephalic appendages, the slender jointed antennules, 

 the biramous character of all the other limbs, and their original phyllopodiforni 

 structure. They differ from Limulus, not only in most of these respects, but 

 also in not having an operculum. From Limulus and all other Arthropods 

 they are distinguished by having compound eyes on free cheek-pieces, which 

 apparently represent the pleura of a head segment that is otherwise lost, 

 except possibly in some forms of stalked eyes and in the cephalic neuromeres 

 of later forms. The most recent discussions as to the affinities of Trilobites 

 are to be found in the papers by Bernard, Kingsley, Woodward, and Beecher, 

 where, from the facts presented, the relationships of these animals with the 

 Crustacea follow as a necessary corollary. 



As to the rank of the Trilobites in a classificatory scheme, there is also 

 much diversity of opinion. They have been long regarded as an order of 

 Crustaceans, but any attempt to include them in this way under higher groups, 

 such as the Entomostraca, Malacostraca, or Merostomata, results in such broad 

 generalities and looseness of definition as to render these divisions of little 

 value. The present state of knowledge of Trilobite structure and develop- 

 ment is in favour of assigning them nothing short of the rank of a 

 sub-class. 



In nearly every particular, the Trilobite is very primitive, and closely 

 agrees with a theoretical Crustacean ancestor. Its affinities are with the 

 known sub-classes of the group, especially their lower orders, but its position is 

 not intermediate. The more primitive characters may be summarised as 

 follows : (1) They are all free marine animals ; (2) they have a definite con- 

 figuration ; (3) the larva is a protonauplius-like form ; (4) the body and 

 abdomen are richly segmented, and the number of segments is variable ; (5) 

 the head is typically pentamerous ; (6) the thorax and abdomen are always 

 distinct, the number of segments in each being variable; (7) all segments 

 except the anal bear paired appendages ; (8) all appendages except antennules 



