Louis De Vos

Comparative study of the choanosome of Porifera: 1. The Homoscleromorpha

The choanoderm and pinacoderm of representatives of the two families of Homoscleromorpha sponges, the Oscarellidae and Plakinidae, have been examined by transmission and scanning electron microscopy. Different fixative procedures have shown the dramatic influence of fixation conditions on the morphology of choanocytes. These two families of sponges have the following morphological features in common: flagellated endopinacocytes with short apical microvilli and basal pseudopods; the presence of a very thin and dense sheet of matrix material which limits the mesohyl. There are, however, only minor differences in the flagellar morphology, granule content, and anchoring system of their choanocytes.

Comparative study of the choanosome of porifera: II. The keratose sponges

The aquiferous system of representatives of the orders Dictyocer‐atida, Dendroceratida, and Verongida has been studied to note its relevance to the systematics of the groups. The volume of the choanocyte chamber, the size and shape of the choanocytes, the number of choanocytes per chamber, the relative development of the mesohyl, and the features of endopinacocytes are estimated from scanning and transmission electron microscopic observations of representatives of most families of the three orders. Although the Dysideidae have a reticulate skeleton and were classified in the order Dictyoceratida, they are actually closer to the Aplysillidae (Dendroceratida) than to dictyoceratids. The anatomy and cytology of the Halisarcidae differ profoundly from those of these three orders and are clearly more closely related to nonkeratose sponges. Some changes in classification lead to a pattern with highly homogeneous orders that clearly differ in their anatomic and cytologic features, which does not support the hypothesis of a common origin of the “keratose” sponges.

Description and significance of a peculiar intradigestive symbiosis between bacteria and a deposit-feeding echinoid

Abstract Individuals of the deposit-feeding echinoid Echinocardium cordatum (Pennant) harbour peculiar irregular to round-shaped nodules in their intestinal caecum. The nodules have a well-defined structure being made of a central detrital core originating from the gut content and surrounded by actively growing bacteria. These bacteria consist of chains of rod-shaped, usually sheathed cells. They form a core-embedding stratified coat, the thickness of which allows estimation of the relative age of the nodules. The selective penetration of the core-forming detritus into the caecal lumen is explained in terms of the fluid properties of the gut sediment. Nodule formation takes place repeatedly within the caecum where newly-arrived potential cores are progressively colonized by bacteria. The oldest nodules tend to be eliminated through the anus of the echinoids. Nodule bacteria appear to be important for the echinoids well-being as their activity presumably results in avoiding the reduction of the intradigestive sediment in the echinoids hindgut.

Functional morphology of the pedicellariae of the asteroid Marthasterias glacialis (Echinodermata)

SummaryMarthasterias glacialis bears two kinds of pedicellariae. The straight pedicellariae are single and occur everywhere on the asteroid body surface except in the ambulacral groove. The crossed pedicellariae are clustered on mobile structures (the rosettes) build around marginal and abactinal spines.Basically, each pedicellaria has a head and a stalk. A skeleton occurs only in the pedicellarial head. It consists of two valves and a basal piece. Muscular bundles are anchored on these skeletal ossicles. The straight pedicellariae have two pairs of adductor muscles (the inner and the outer adductors) and one pair of abductor muscles, these latter being weakly developed. Longitudinal muscle fibers occur all along the stalk of straight pedicellariae. The crossed pedicellariae have two pairs of adductor muscles (the distal and the proximal adductors) and two pairs of abductor muscles (the distal and the proximal abductors). The proximal adductors of crossed pedicellariae are homologous to the stalk muscles of straight pedicellariae.The pedicellariae are able to react to direct and indirect tactile stimuli. There is a great deal of individual variation among pedicellarial responses. Moreover, the reactions occur at random and lack coordination. The seemingly aberrant behavior of the pedicellariae is interpreted as a preventive activity that protects the asteroid body surface against unwanted materials and organisms.