RELATIONSHIPS BETWEEN ARTHRODIAL MEMBRANE FORMATION AND ADDITION OF SETAE TO SWIMMING LEGS 1-4 DURING DEVELOPMENT OF DIOITHONA-OCULATA, RIDGEWAYIA-KLAUSRUETZLERI, PLEUROMAMMA-XIPHIAS, AND TEMORA-LONGICORNIS (COPEPODA)

Relationships between arthrodial membrane formation and the addition o f setae to the exopod and endopod of swimming leg 3 of the cyclopoid c opepod Dioithona oculata are derived from formation homologies during development. Based on this information, these relationships are inferr ed for the remaining swimming legs of the cyclopoid and the swimming l egs of three calanoids, Ridgewayia klausruetzleri, Pleuromamma xiphias , and Temora longicornis. Most of the variation in setal numbers for a leg among the four copepods is expressed in the distal segment of eac h ramus. The most striking differences in architecture result from a d elay or failure in expression of the arthrodial membranes which separa te the proximal segment or the middle segment of the ramus from the di stal segment. Inferring a composite model for relationships between ar throdial membrane formation and addition of setae to the exopod and en dopod of a swimming leg is complicated by differences in swimming leg 1 and the remaining swimming legs which includes an earlier allocation of the proximal seta of the middle (third) segment to both rami of sw imming leg 1. These developmental patterns are presumed to be derived, and the endopod of all thoracopods of the ancestral copepod immediate ly after the transformation of the leg bud is presumed to have been 2- segmented with one medial sera on the proximal segment.