L'Origine Protocéphalique De La Carapace Chez Les Thermosbaenacés, Tanaidacés, Cumacés Et Stomatopodes

Abstract

AbstractThis study deals with the formation of the carapace in Malacostraca which have the last thoracic segments free. In all of them, the three gnathal cephalomeres are dorsally split open whereas this phenomenon affects only a few thoracomeres: the first and part of the second in the thermosbaenacean Monodella argentarii; the first and the second in the tanaidaceans Allotanais hirsutus and Apseudes spectabilis; the first, second and third in the cumacean Heterocuma intermedia, the last two being moreover stretched. The pseudozoean stages of Stomatopoda are slighty different: only the first thoracomere is partly split open and connected by epimera to the sides of the carapace which extends as far as the fourth thoracomere; the last four thoracomeres are exposed. The organization differs in the protocephalic area. In Thermosbaenacea, Tanaidacea and Cumacea which have sessile eyes or arc blind, the protocephalon is undivided. In Stomatopoda which have stalked eyes, the ocular and antennular segments are differentiated as previously observed in other Podophthalmata; that allows to observe that the carapace is produced by the sole antennal segment. Hence, the origin of the carapace is the same in all Malacostraca: it is a dorsal expansion of the protocephalic area (antennal segment when differentiated) which splits and pushes aside the gnathal cephalic tergites and a variable number of thoracic tergites. The tergites play absolutely no role in this formation, either by fusion or by coalescence.