Cirripedia (barnacles) constitute a crustacean monophyletic taxon which is very well defined by several synapomorphies. In particular, all cirripedes are composed of six thoracic segments, but are devoid of any complete abdominal segment. This body plan is preserved in the adult in non-parasitic groups, while the parasitic rhizocephalan cirripedes completely lose arthropodian segmentation at the adult stage. These traits make them a particularly favourable model for studying the formation and maintenance of segmental identity. For the above reasons, it seemed worthwhile to look at the segmentation gene engrailed in a cirripede. A complete engrailed.a cDNA was isolated from larvae of the rhizocephalan cirripede Sacculina carcini. Its expression was monitored during larval development by use of the monoclonal antibody MAb4D9 directed against the Drosophila homologous proteins. The Sacculina engrailed.a gene is expressed during the second and third larval stages in stripes within a posterior area corresponding to the presumptive trunk segments. Surprisingly, these stripes appear in a posterior to anterior sequence. Six engrailed.a stripes characterize the thoracic segments of the cirripedean ground plan.

Experimental infestations of young Carcinus with Sacculina carcini indicate that the ventral ganglionic mass (VGM) is the first "target" of the parasite roots into the host tissues. As in pubescent crabs naturally parasitized, either with or without an external visceral sac, the roots penetrate and invade that area of the central nervous system during the first month of infestation.Ultrastructural study of the developing roots, in contact with the VGM, leads to the conclusion that apart from a few embryonic characters, the root cells show a cytostructure similar to the one that has been observed in the roots naturally parasitizing pubescent crabs, whether the external visceral sac is present or not.In the two cases of infestation the roots which cross the neurolemma bring about a marked disorganization of ganglia in consequence of the alteration of the neuroglia, neuropiles and neurosecretory areas.An action of the parasite at a distance, i.e. in the absence of contact of the roots with the ganglia, also occurs. It is especially marked as early as 1 month after infestation by a degeneration of the secretory perikarya.A thin sheath of connective tissue is sometimes visible between the growing roots and the VGM of juvenile crabs. However, the existence of that tissue cannot be definitely attributed to a host defense reaction.No effect of the developing parasite on the already differentiated external sex characters of the host has been noticed within the limits of the experiments.

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