Structure of the Nemertine Integument with Consideration of Its Ecological and Phylogenetic Significance

SVNOPSIS. The phylum Nemertina, with more than 800 described species, occupies a wide range of habitats and figures prominently in theories of bilaterian evolution. Integuments of nemertines, from a number of habitats and representing the major orders, are compared and the general cell types constituting the epidermis are reviewed and defined. The general construction and cell demography of the integument are more or less characteristic for each order, but are not readily generalized to lower taxonomic levels. Similarly, the general structure of the integument has correlations with broad environments, but there are few conspicuous, uniquely adaptive, morphological specializations; e.g., the adhesive plate and ciliary bristles of interstitial nemertines, the tube-forming cells of Carcinonemertes epialti and various palaeonemertines, the epidermis of the pilidium larva. It is proposed that there are three generalized types of integumentary glandular cells, mucous, serous and bacillary, and that these and the ciliated cells vary only modestly across taxonomic and environmental lines. However, it is also proposed that the major structural variations of the integument not only are diagnostic for the orders but also suggest and correlate with two major evolutionary radiations of the extant nemertines, one epibenthic, the other infaunal—in that sequence. Similarities of the nemertine integument to that of turbellarians are superficial or at best plesiomorphous; i.e., they are also found in other taxa.