The fine structure of the cuticle of kamptozoans is similar to that of annelids

Abstract

The Kamptozoa is a small group of invertebrates whose position on the phylogenetic tree of the Bilateria is still unclear. The relationship between kamptozoans and other bilaterians might be at least partially clarified by detailed morphological analysis. The cuticle has a distinctive structure in different groups of Bilateria, and its study is required for comprehensive comparative analysis. The ultrastructure of the cuticle of two colonial kamptozoans Barentsia discreta (Busk, 1886) and Pedicellina cernua (Pallas, 1774) was studied by transmission electron microscopy. Two types of cuticle were found. The atrial cavity and the frontal surfaces of the tentacles are coated with a glycocalyx that is penetrated by epidermal microvilli. This type of cuticle is widespread in many invertebrate phyla and appears to be a primitive stage of cuticle evolution. The cuticle of other regions of the kamptozoan colony includes a fine external osmiophilic layer and a thick fibrous layer. In the rigid parts of the colony (the peduncle of B. discreta and the stolon of both species), the cuticle has an additional thick homogeneous layer. The cuticle structure correlates with the degree of musculature. The microvilli of the epidermal cells pass through the cuticle and end with capitate swellings. Comparative analysis of the cuticle fine structure revealed the presence of many morphological features that are similar in kamptozoans and annelids. At the same time, the kamptozoan cuticle has little in common with the cuticle of bryozoans and cycliophorans, groups that have traditionally been regarded as the closest relatives of kamptozoans.