The present study characterizes the embryonic development of Aegla platensis Schmitt, 1942 by means of optical microscopy and electron microscopy. The sequence of morphological changes observed in the egg allowed to describe ten embryonic stages: the unsegmented egg, cleavage, blastula, gastrula, nauplius, metanauplius, early zoea, late zoea, decapodid, and juvenile. Cleavage was centrolecithal and the blastula was regular. The nauplius stage was transient. The development of the gut could be observed during the zoea stages. At the late zoea stage the posterior midgut was well developed showing chromophobous cells. The dark eye pigmentation started to develop at the early zoea stage and was completed at the decapodid stage. Newly hatched juveniles exhibit some yolk incorporationin the cephalothoraxand do not need food during the rst days after hatching. The morphological changes observed during the development of this anomuran are discussed in relation to patterns of development reported for other allegedly allied decapods. ZUSAMMENFASSUNGIn der vorliegenden Arbeit wird die Embryonalentwicklung von Aegla platensis Schmitt, 1942 licht-und elektronenmikroskopischuntersucht.Aufgrund der Abfolge morphologischerVeränderun-gen im Ei können 10 Embryonalphasen unterschieden werden: ungefurchtes Ei, Furchungteilungen, Blastula, Gastrula, Nauplius, Metanauplius, frühe Zoëa, späte Zoëa, Decapodid und Juvenil. Die Furchung war centrolecithalmit regulärer Blastula und transitorischemNauplius. Die Darmentwicklung konnte während der Zoëa-Phase beobachtet werden. In der späteren Zoëa-Phase zeigte sich der hintere Mitteldarm gut entwickelt mit chromophoben Zellen. Die dunkle Pigmentierung der Augen begann während der frühen Zoëa-Phase und war in der Decapodid-Phase abgeschlossen. Bei frisch geschlüpften Juvenilen be ndet sich Dotter im Cephalothorax, so daß in den ersten Tagen nach dem Schlüpfen keine Nahrung benötigt wird. Die morphologischen Veränderungen im Verlauf der Entwicklung dieses Vertreters der Anomura werden mit denen bei anderen, vermeintlich verwandten Decapoden verglichen. © Koninklijke Brill NV, Leiden, 2003 Crustaceana 76 (1): 13-25

The sub-chronic (28-56 days) effects of exposure to low concentrations of cadmium (Cd; 0Á05, 0Á25, 0Á50 and 2Á50 mg l À1 ) shortly following fertilization on embryos, larvae and juvenile rainbow trout Oncorhynchus mykiss were examined. Premature hatching occurred at lower concentrations (0Á05 and 0Á25 mg l À1 Cd), however, delayed hatching was seen in the 2Á50 mg l À1 Cd group, with >90% of hatching occurring on the last day of the hatching period. Larval growth was negatively affected by Cd exposure in a concentration-dependent manner. Larvae exposed to 2Á50 mg l À1 Cd were 13Á9 AE 0Á8% shorter in total length (L T ) and weighed 22Á4 AE 3Á5% (mean AE S.E.) less than controls at the end of the exposure period. Plasma sex steroid concentrations (oestradiol in juvenile females and 11-ketotestosterone in juvenile males) were elevated (four-to 10-fold over controls) in exposed fish in both males and females, following 28 days of exposure to 0Á05, 0Á25 and 0Á50 mg l À1 Cd, respectively. These results suggest that environmentally realistic concentrations (in the mg l À1 range) of Cd can affect the development of O. mykiss impacting embryos, larvae and juvenile fish.

The embryonic development of freshwater triclads is mainly known from studies of species of Dendrocoelum, Planaria, Polycelis, and, more recently, Schmidtea. The present study characterizes the development of Girardia tigrina (Girard, 1850) by means of optical microcopy using glycol methacrylate semi-thin sections. 94 cocoons were collected in the period from laying to hatching, with intervals of up to twenty-four hours. The sequence of morphological changes occurring in the embryo permitted the identification of nine embryonic stages. At the time of cocoon laying, numerous embryos were dispersed among many yolk cells, with a rigid capsule covering the entire cocoon. In the first stage (approx. up to 6 hours after cocoon laying), yolk cells and embryonic cells showed random distribution. Stage II (between 12 and 24 hours after cocoon laying) is characterized by aggregates of blastomeres, which later aggregate forming an enteroblastula. Approximately 2 days after cocoon laying (stage III), formation of the embryonic epidermis and embryonic digestive system took place, the latter degenerating during the subsequent stage. Stage V (until the fourth day) is characterized by the formation of the definitive epidermis. Between 4 and 6 days after laying, organogenesis of the definitive inner organs starts (stage VI). Approximately 14 days after laying (stage IX), formation of the nervous system is completed. At this stage, the embryo shows similar characteristics to those of newly hatched juveniles. The hatching of Girardia tigrina occurs in the period between twelve to twenty-two days after cocoon laying.Keywords: developmental stages, morphology, optical microscopy, Dugesiidae, freshwater planarians. , 1850) (Platyhelminthes, Tricladida, Paludicola) Desenvolvimento embrionário de Girardia tigrina (Girard ResumoO desenvolvimento embrionário dos tricladidos é conhecido, fundamentalmente, por estudos realizados em espécies de Dendrocoelum, Planaria, Polycelis e, mais recentemente, Schmidtea. O presente estudo descreve o desenvolvimento embrionário de Girardia tigrina (Girard, 1850), a partir de análises realizadas em cortes histológicos seriados e semifinos de glicol-metacrilato, ao microscópio óptico. Noventa e quatro casulos foram coletados no período entre a postura e a eclosão, em intervalos de até vinte e quatro horas. A seqüência das modificações morfológicas no embrião permitiu a identificação de nove estágios embrionários. Na postura dos casulos, envoltos por uma cápsula rígida, observam-se numerosos embriões dispersos entre grande quantidade de células vitelinas. No estágio I (aproximadamente até 6 horas após a postura), as células vitelinas e as embrionárias mostram uma distribuição aleatória. O estágio II (entre 12 e 24 horas após a postura) caracteriza-se pela formação de agrupamentos de blastômeros, os quais posteriormente formam uma enteroblástula. Aproximadamente dois dias após a postura (estágio III), ocorre a formação da epiderme e do sistema digestivo embrionário, sendo que este último degenera ...