The Protonephridia of the Arctic KinorhynchEchinoderes aquilonius(Cyclorhagida, Echinoderidae)

Allometric growth in meiofaunal invertebrates: do all kinorhynchs show homogeneous trends?

Allometry determines relevant modifications in metazoan morphology and biology and is affected by many different factors, such as ontogenetic constraints and natural selection. A linear mixed model approach and reduced major axis regression were used to explore evolutionary interspecific allometric trends between the total trunk length and the lengths of the segments and spines in the phylum Kinorhyncha at three taxonomic levels: the whole phylum, the class and the family. Statistically significant results were found in all the trunk segments, meaning that these body units grow proportionally correlated with the body, contrary to the results obtained for the spines. Developmental and morphophysiological constraints could lead to negative allometry in the first and last segments, because these body regions in kinorhynchs are essential to the implementation of some of the main biological functions, such as feeding and locomotion. The differential arrangement of cuticular appendages between the taxonomic groups considered seems to cause different evolutionary trends, because positive allometry may appear if a segment requires more space to accommodate a large number of organs and appendages, and vice versa. The presence of sexual dimorphism could also define positive allometry of a segment, owing to the need to harbour the sexually dimorphic appendages and their associated structures.

Fine Structure of the Integumentary Cuticles and Alimentary Tissues of Pycnophyid Kinorhynchs Pycnophyes oshoroensis and Kinorhynchus yushini (Kinorhyncha, Homalorhagida)

Integumentary and alimentary tissues were ultrastructurally examined in two pycnophyid kinorhynchs, Pycnophyes oshoroensis and Kinorhynchus yushini, to elucidate some aspects of their ecology. The body is entirely enveloped by an epicuticle layer with no gaps between cuticle plates and joints. The cuticular layer has a structure dense enough to prevent invasion by foreign organisms. The cuticular surface is overlaid by a mucus layer that may form a hydrophilic surface. The alimentary contents were heterogeneous, probably including some cellular components, such as chloroplast-like structures. Kinorhynchs likely break down food particles in the pharyngeal bulb by pressing it between the cuticulated epithelia. The pharyngeal crown was located in front of the pharyngeal bulb and had a thick wall with a striated sub-structure. Contraction of the pharyngeal bulb probably increases the internal pressure of the pharyngeal crown; this may be one reason for the thick wall of the pharyngeal crown. Nutrients appear to be taken up by midgut epithelial cells through both absorption via microvilli and endocytosis. Additionally, sperm tails in the testis of P. oshoroensis have unusual axonemes; i.e., an 18+9+2 pattern.

Comparative morphology of serotonergic-like immunoreactive elements in the central nervous system of kinorhynchs (Kinorhyncha, Cyclorhagida)

Cycloneuralian taxa exhibit similar organ system architectures, providing informative characters of metazoan evolution, yet very few modern comparative descriptions of cellular and molecular homologies within and among those taxa are available. We immunolabeled and characterized elements of the serotonergic nervous system in the kinorhynchs Echinoderes spinifurca, Antygomonas paulae, and Zelinkaderes brightae using confocal laser scanning microscopy. Fluorescent markers targeting DNA were combined with observations of auto-fluorescent structures to guide interpretations of the internal and external anatomy in each species. Results show a common pattern of the central nervous system with a circumenteric brain divided into ring-shaped anterior and posterior neuronal somata and a central neuropil connected to a multi-stringed, longitudinal ventral nerve cord. Structural similarities and differences in the nervous systems of these species were observed and described, stressing the incomplete ring nature of the anterior region of the kinorhynch brain, the functional relationship between the brain and the movable introvert, and the number and arrangement of nerve strings and somata of the ventral nerve cord. The ventral cord ends in two ventrolateral cell bodies in E. spinifurca, and forms a terminal loop associated with a midterminal spine in A. paulae and Z. brightae. The possible functional and phylogenetic significance of these features and arrangements are discussed.

Ultrastructure of the protonephridia of larvalRugiloricuscf.cauliculus, maleArmorloricus elegans, and femaleNanaloricus mysticus(Loricifera)

The protonephridial system of several Loricifera was studied by transmission electron microscopy. A larval specimen of Rugiloricus cf. cauliculus possesses two protonephridia, which are "capped" frontally by a compact mass of still undifferentiated gonadal cells. Each protonephridium consists of four monociliary terminal cells and four canal cells with a diplosome but no cilia. Because of incomplete series of sections and unsatisfactory fixation, the outleading cell(s) could not be detected. In a male specimen of Armorloricus elegans, each gonad contains two protonephridia that open into the gonadal lumen. Each protonephridium consists of two monociliary terminal cells, each forming a filter, two nonciliated canal cells, and two nephroporus cells. The protonephridial lumina of the latter cells fuse to one common lumen, which unites with the gonadal lumen. Preliminary observations on the protonephridia of a female Nanaloricus mysticus reveal a more complicated arrangement of interdigitating terminal and canal cells. One or two terminal cells form their own individual filter or four cells form a common compound filter. The cilium of the terminal cells of all species investigated are surrounded by a palisade of nine microvilli that support the filter barrier made of an extracellular matrix. An additional filter diaphragm could be traced between the pores in the cell wall of each terminal cell of A. elegans. The urogenital system of the Loricifera differs from that of the Priapulida in that the protonephridia of the former are completely integrated into the gonad, whereas the excretory organs of the latter open into the urogenital duct caudally of the gonads.

Cuticular structures and epidermal glands of Echinoderes cantabricus and E. hispanicus (Kinorhyncha, Cyclorhagida) with special reference to their taxonomic value

The body wall of two species of kinorhynchs, Echinoderes cantabricus and E. hispanicus, was examined with transmission and scanning electron microscopy, to determine accurately the nature, arrangement, and consistency of characters used for taxonomic purposes. Structural details of cuticular hairs, pectinate fringes, spines, tubules, and different cuticular scars are described and discussed. Two kinds of epidermal glands, types 1 and 2, are characterized according to the nature of their secretory products, their exact location and distribution in the trunk segments, and their possible value as taxonomic characters. The generally assumed function of tubules for the release of secretory product is analyzed and finally rejected in light of the different distribution of tubules and glands along the trunk and because of the absence of a clear anatomic relation between both structures. The cuticular features studied can be useful for taxonomic purposes because of their consistency, but some of them are difficult to access and evaluate and therefore must be used with caution.