Immunocompetent cells requisite for graft rejection in Lineus (Invertebrata, Nemertea).

Antecerebral ends from donors of one Lineus species (L. sanguineus) were grafted onto bispecific recipients previously constructed from two other Lineus species (denoted L. ruber----L. lacteus because the anterior component of chimeras was from L. ruber and the posterior component was from L. lacteus) and onto monospecific controls. Histological examination of areas where the tissues from L. sanguineus and L. ruber had been brought into contact by grafting always showed, at early stages, (6 to 20 days postgrafting), a great deal of difference depending upon whether the recipients were monospecific L. ruber or bispecific L. ruber----L. lacteus: only in grafts onto the former was there lysis of gland cells, connective tissue, muscular fibers, and finally epidermis. We attribute this lytic process to a strongly and rapidly cytotoxic action of lymphocyte-like cells from the L. ruber intestinal segment and the absence of lysis during the same stage in grafts onto composite recipients and monospecific L. lacteus to weak, delayed actions of immunocytes from the L. lacteus intestinal segment. Subsequent phagocytosis of material from lysed cell of grafts in the process of being rejected was effected by wandering amebocytes usually involved in destruction of degenerating "self" components, as in oosorption and resorptive processes after fasting. This work supports the existence of immunocytes at an early phylogenetic level.