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

Abstract

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.

Redescription and Taxonomic Reassessment of Nemertellina minuta Friedrich, 1935,sensuYamaoka, 1940 (Nemertea, Hoplonemertea, Monostilifera)

“…4) is 12-30 µm thick, but with few acidophilic gland cells in the ventral pre-cerebral regions of the head (Figs 4-7) compared with other regions of the body. Basally the epidermis possesses a distinct basal-cup zone, as described by Norenburg (1985). The dermis is only 1-7 µm thick but distinct (Fig.…”

Phylogenetic Relationships among Higher Nemertean (Nemertea) Taxa Inferred from 18S rDNA Sequences

“…But this view has been questioned (e.g., Norenburg, 1985;Turbeville, 1991) and is not supported by alternative cladistic analyses of morphology (Eernisse et al, 1992;Schram, 1991;Zrzavy et al, 1998). The latter analyses suggest that the nemerteans are most closely related to spiralian (Protostome) coelomates.…”

“…Turbeville (1991) furthermore interprets the presence of unique secretory bodies (rhabdoids or pseudocnids) in palaeo-and heteronemerteans as a possible synapomorphy. Also, Norenburg (1985Norenburg ( , 1993 concluded that the palaeonemerteans form a nonmonophyletic group and that some palaeonemertean taxa (e.g., Hubrechtella) were more closely related to heteronemerteans than to other palaeonemerteans. We, however, have not been able to establish any well-supported sister group relationships between the palaeonemertean and the heteronemertean species included using 18S rDNA sequences.…”

Nemertea

“…The defi nite, adult epidermis consists of multiciliated cells, several types of gland cells, basal granular cells, and basal neurites originating from subepidermal neurons constituting a pseudostratifi ed epithelium in paleonemertean and heteronemertean species (Turbeville 1991 ). In Hoplonemertea, due to the presence of a basal cup cell layer, the pseudostratifi ed appearance of the epidermis is even more pronounced (Norenburg 1985 ;Turbeville 1991 ). The heteronemertean dermis is formed late in development, i.e., after metamorphosis, by some gland cells sinking underneath the level of the ciliated epidermis cells.…”