IIO THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



Cambrian Branchiopoda are more closely allied to the Trilobita than are the modern ones, 

 but still the subclass is not so closely related to that group as has been thought. Modern 

 Apus is certainly much less like a trilobite than has been supposed, and very far from being 

 primitive. The Branchiopoda of the Middle Cambrian could have been derived from the 

 trilobites by the loss of the pleural lobes, the development of the posterior margin of the 

 cephalon to form a carapace, and the loss of the appendages from the abdominal segments. 

 Modern branchiopods can be derived from those of the Middle Cambrian by the modifica- 

 tion of the appendages through the reduction of the endopodite and exopodite and the 

 growth of the endites and exites from the proximal segments. 



Carpenter (1903, p. 334), from his study of recent crustaceans, has already come to 

 the conclusion that the Branchiopoda are not the most primitive subclass, and this opinion 

 is strengthened by evidence derived from the Trilobita and from the Branchiopoda of the 

 Middle Cambrian. 



COPEPODA. 



The non-parasitic Eucopepoda are in many ways much nearer to the trilobites than any 

 other Crustacea. These little animals lack the carapace, and the body is short, with typi- 

 cally ten free segments and a telson bearing caudal furcze. The head is composed of five 

 segments (if the first thoracic segment is really the fused first and second), is often flat- 

 tened, and lacks compound eyes. Pleural lobes are well developed, but instead of being 

 flattened as in the trilobite, they are turned down at the sides or even incurved. A labrum 

 is present. 



The antennules, antenme, and mandibles are quite like those of trilobites. The anten- 

 nules are very long and made up of numerous segments. The antennae are biramous, the 

 junction between the coxopodite and basipodite is well marked, and the endopodite consists 

 of only two segments. 



The mandibles are said to "retain more completely than in any other Crustacea the 

 form of biramous swimming limbs which they possess in the nauplius." The coxopodites 

 form jaws, while both the reduced endopodite and exopodite are furnished with long setae. 

 The maxillulse are also biramous, but very different in form from those of the trilobite, 

 and the maxillae are phyllopodan. 



The first thoracic limb is uniramous and similar to the maxillae, but the five following 

 pairs are biramous swimming legs with coxopodite, basipodite, exopodite, and endopodite. 

 Both the exopodite and endopodite are shorter than in the trilobites, but bear setae and spines. 



The last pair of thoracic limbs are usually modified in the male into copulatory organs. 

 In some females they are enlarged to form plates for the protection of the eggs, in others 

 they are unmodified. In still others they are much reduced or disappear. The abdomen 

 is without appendages. 



The development in Copepoda is direct, by the addition posteriorly to the larval form 

 (nauplius) of segments, and the appendages remain nearly unmodified in the adult. 



Altogether, the primitive Copepoda seem much more closely allied to the Trilobita than 

 any other modern Crustacea, but unfortunately no fossil representative of the subclass 

 has been found. This is not so surprising when one considers the habits and the habi- 

 tat of most of the existing species. Many are parasitic, many pelagic in both fresh 

 and marine waters, and many of those living on the bottom belong to the deep sea or fresh 

 water. Most free-living forms are minute, and all have thin tests. 



