ISSN 0012-4966, Doklady Biological Sciences, 2006, Vol. 406, pp. 76–78. © Pleiades Publishing, Inc., 2006. Original Russian Text © A.A. Kotov, 2006, published in Doklady Akademii Nauk, 2006, Vol. 406, No. 3, pp. 422–424. 76 Cladocera (Crustacea: Branchiopoda) are a group of microscopic animals common for continental waters. Different cladocerans are objects of various studies, some of them using recent methods; however, some important details of their morphology and anatomy remain unclear [1]. A series of studies on the compara- tive morphology and ontogeny of cladocerans was recently performed [7–10]. Here, I describe a fundamentally new approach to homologization of the caudal tagma of cladocerans, other representatives of the group Cladoceromorpha that are the closest to cladocerans, and archaic branchi- opods. According to recent data, four orders of cladocer- ans, namely, Anomopoda, Ctenopoda, Onychopoda, and Haplopoda, form a monophyletic clade within the group Diplostraca [9, 10]. Besides cladocerans, they include three more orders (Spinicaudata, Laevicaudata, and Cyclestherida), which were earlier combined into the group Conchostraca. Note that Cyclestheridae is a sister group of cladocerans; together, they form the group Cladoceromorpha [9, 12]. Traditionally, postabdominal claws of Diplostraca were regarded as homologues of the rami of the caudal furca of archaic branchiopods, e.g., the currently exist- ing order Anostraca and the Cambrian Rehbachiella [13]. Furcal rami in crustaceans are appendages of the telson, an unsegmented caudal tagma. However, Bow- man [4] doubted the homology of the claws of “concho- stracans” to the furca and suggested that they should be regarded as uropods, i.e., appendages of the posterior abdominal segment, assuming that telson was reduced in “conchostracans.” The results of recent studies on the embryonic development in cladocerans from orders Anomopoda and Ctenopoda [7, 8] cast doubt on the hypothesis that their postabdominal claws were homologous to the fur- cal rami. In embryos of Anomopoda and Ctenopoda, a furrow is formed on the posterior part of the body soon after the segmentation of thorax is completed. In my opinion, this furrow separates the rami of the furca (as in the nauplius of Anostraca [5] or the fossil Reh- bachiella [13]). The rudiments of postabdominal claws of cladocerans appear considerably later, about the time when the rudiments of setae are formed on the antennae and limbs of the embryos, and all tagmae of the body are well separated from one another (also similarly to the setae on the furca of archaic branchiopods). The formation of the claws simultaneously with setae, rather than during body segmentation, suggests that the claws are more likely to be homologous to simple setae than to telson appendages. Before the embryonic molting (the stage next to that described above), postabdominal claws are differenti- ated in an incurved state in Anomopoda and Ctenopoda [7, 8]. Their formation before the molting of adult ani- mals occurs in precisely the same way (Fig. 1), and the initiation in an incurved state is characteristic of setae on the appendages of crustaceans [6]. The furcal rami in Anostraca are formed in a different way, whereas each of the numerous setae of the furca is differentiated in an incurved state, as was clearly demonstrated by Claus as early as in 1873 [5] and confirmed later by other researchers. In Leptodora kindti (the only species of the order Haplopoda), the cladoceran that is the most different from others in this respect, the postabdominal claws of adult animals are massive and are attached to the post- abdomen without a joint. In embryos of this animal, however, claws appear as thin setae attached to the body with joints [10]. During the molting of adult ani- mals, the claws are differentiated in an incurved state [14]. Thus, the specific claws of the Leptodora have also originated from the furca setae, as in other cla- docerans. The homology of the postabdominal claws of Cla- doceromorpha to the simple setae on the furca of Anos- traca is especially clearly demonstrated in the example On the Homology of the Caudal Tagma in Cladoceromorpha (Crustacea: Branchiopoda) A. A. Kotov Presented by Academician D.S. Pavlov June 1, 2005 Received June 8, 2005 DOI: 10.1134/S0012496606010212 Severtsov Institute of Ecology and Evolution, Russian Academy of Sciences, Leninskii pr. 33, Moscow, 119071 Russia GENERAL BIOLOGY