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In this study we describe the site and moment of histospecific differentiation in developmental stages of the annelid Platynereis dumerilii by use of biochemical markers. The monoclonal antibody (mab) OI7 and uncloned hybridoma supernatants (pAb's) OI8, OI10, OI46 and OI69 recognize neural antigens that appear asynchronously during development. By an enzymatic test, acetylcholinesterase (AChE) was found specific for nervous tissue as well. The patterns of neural structures labelled by antibodies differ, however, from those revealed by AChE staining. Experimental inhibition of transcription (with actinomycin D) and of translation (using puromycin) demonstrate that the expression of histospecific neural markers depends on both zygotic transcription and subsequent translation. The mAb OI64 labels epidermal (and neural) gland cells. The antibody 4D9, raised against the engrailed protein of Drosophila, labels single rows of ciliated cells at the posterior border of segments.

The coccidian, Coelotropha durchoni, manages to develop in its host, the polycheate annelid, Nereis diversicolor, because of its ability to circumvent the host's internal defence system. First, it avoids phagocytes by penetrating other cells, principally eleocytes and muscular cells, where it undergoes a phase of intracellular development. After becoming extracellular, a thick coat protects it from being attacked by granulocytes. This coat then breaks and is shed from its surface to permit fertilization. Parasites that have lost their coats and remain unfertilized are surrounded with granulocytes and destroyed by encapsulation. A strict hormonal correlation exists between the biological cycles of the parasite and its host. Thus, the mature spores of the coccidian parasite are disseminated when the worm lays its gametes by rupture of teguments. C. durchoni and N. diversicolor have established a biological equilibrium that permits mutual survival of both partners and constitutes a simple model for the study of the host-parasite relationship.

The presence in the marine worm Nereis diversicolor of a low molecular mass protein with the capacity to bind cadmium has been previously demonstrated [5]. Poly(A) + -mRNA were extracted from coelomocytes of Nereis diversicolor and were translated either in vitro , using a rabbit reticulocyte lysate, or in vivo into Xenopus laevis oocytes. Analysis of synthesized polypeptides by enzyme-linked immunosorbent assay (ELISA) and by Western blotting, using a specific monoclonal anti-MP II antibody, showed that this metalloprotein was translated both in in vitro and in vivo translation systems, with an apparent molecular mass of 11–13 kDa. Two other products, with 26.5 and 28 kDa molecular mass, cross-reacted with the monoclonal anti-MP II antibodies. The present work confirms that coelomocytes are sites of important synthesis of MP II-mRNA.

The growing stages of Aggregata eberthi, in the cuttlefish on the one hand (sexual generation), and in the crab on the other hand (asexual generation), are investigated.