Acta Zoologica

Benito, Jesús; Fernández, Isabel; Pardos, Fernando

doi: 10.1111/j.1463-6395.1993.tb01224.xpmid: N/A

Abstract The hepatic region of Glossobalanus minutus is characterized by deep foldings of the dorsal side of the gut epithelium which affect the neighbouring tissues and structures: coelomic spaces, musculature and epidermis. The following cell types of the gut epithelium are described: vacuolated cells, undifferentiated cells, two types of mucous cells and two types of granular secretory cells. The nature and function of the different cell types are discussed. Data on the general ciliation and subepithelial nerve plexus of the gut epithelium are also given, with special mention of a possible neuroendocrine secretion towards the subjacent blood spaces. A well‐developed blood sinus (gut sinus) lies between the gut and the visceral peritoneum. The ultrastructural features of the gut epithelium and its close association with the blood sinus point to an absorptive function. The coelomic cavity is reduced to a narrow space limited by two peritoneal sheets (visceral and parietal) of myoepithelial nature. Amoebocyte‐like cells (coelomocytes) occur free in the coelomic fluid, and muscular, unicellular bridges are attached to both peritoneal walls across the coelomic space. The dorsal epidermis follows the gut foldings and is formed by flat, overlapping cells. The present observations are compared with previous histological, histochemical and ultrastructural data.

Balser, Elizabeth J.; Ruppert, Edward E.

doi: 10.1111/j.1463-6395.1993.tb01225.xpmid: N/A

Abstract The spongy body of Davidaster rubiginosa, D. discoidea, and Comactinia meridionalis, is an axial haemal plexus consisting of two structurally similar, but positionally distinct, regions: an oral circumesophageal part and an aboral part which lies lateral to the axial organ. The axial organ is a large axial blood vessel which is infiltrated by hollow cellular tubes lined with monociliated epithelial cells. The spongy body plexus is a tangle of small blood vessels overlain by podocytes and myocytes. The spongy body and the axial organ are situated in the axial coelom, which is confluent with the perivisceral coelom, the water vascular system, and the parietal canals. The parietal canals open to the exterior via ciliated tegmenal ducts and surface pores. The crinoid spongy body is morphologically similar to the axial gland of asteroids, ophiuroids, and echinoids (AOE). Although the axial glands of these three classes of echinoderms are mutually homologous structures, the homology of the crinoid spongy body and the AOE axial gland is questionable because of differences in organization and developmental origin. Alternatively, the crinoid spongy body may be homologous to asteroid gastric haemal tufts, which are podocyte‐covered blood vessels suspended in the perivisceral coelom. The functional organization of the spongy body suggests a filtration nephridium and predicts an excretory function. An alternative hypothesis is that the spongy body is a site of nutrient transfer from the blood vascular system to the perivisceral coelom.

Manni, Lucia; Zaniolo, Giovanna; Burighel, Paolo

doi: 10.1111/j.1463-6395.1993.tb01226.xpmid: N/A

Abstract The formation and cytodifferentiation of egg envelopes were studied at the ultrastructural level in blastozooids of Botryllus schlosseri. The process was divided into five recognized stages of oogenesis. First, the small young oocytes (stage 1) are contacted by scattered cells (primary follicle cells—PFC) which adhere to the oolemma at several junctional spots. PFC extend all around the growing oocyte, acquire polarity, and form a layer covered externally by a thin basal membrane (stage 2). At stage 3 isolated cells are recognizable between the PFC layer and oocyte. They never form junctions with the oocyte and represent prospective inner follicle cells (IFC) and test cells (TC), the latter being progressively received in superficial depressions in the oocyte. The layer of PFC, which maintains junctions with the oolemma, represents prospective outer follicle cells (OFC). PFC are considered to be the source of the three cellular envelopes because a contribution from mesenchymatous elements was not observed. At the beginning of vitellogenesis (stage 4), the vitelline coat (VC) becomes recognizable as a loose net covering the oocyte and TC. It is crossed by the oocyte microvilli and OFC projections which meet and form numerous small junctional plaques, some of them resembling gap junctions. IFC, VC and TC show marked signs of differentiation with approaching ovulation. OFC differentiate completely before ovulation (stage 5) and are engaged in intense synthesis of proteins which may be transferred and taken by endocytosis into the oocyte for yolk formation. Experiments with injected horseradish peroxidase also revealed that proteins present in the blood may reach the oocyte via the intercellular pathway, overcoming OFC and IFC. The possible roles of all the egg envelopes are discussed.

Schuchert, Peter

doi: 10.1111/j.1463-6395.1993.tb01227.xpmid: N/A

Abstract The phylum Placozoa contains the most simply organized multicellular animals: they have a thick lower epithelium and a thin upper epithelium with some single cells sandwiched between these layers. Placozoans are generally considered to have no nerve cells or specialized sensory cells. However, the results of this investigation demonstrate the existence of a specific set of cells which react with antibodies against the neuropeptide family RFamide. The RFamide‐positive cells of Trichoplax adhaerens are positioned at some distance along the margin of the animal.

Justine, Jean‐Lou; Afzelius, Björn A.; Malmberg, Göran; Mattei, Xavier

doi: 10.1111/j.1463-6395.1993.tb01228.xpmid: N/A

Abstract Spermiogenesis was studied by transmission electron microscopy in the acanthocotylid monogeneans Myxinidocotyle californica (from Eptatretus stoutii) and Acanthocotyle lobianchi (from Raja clavata). In Myxinidocotyle and Acanthocotyle, the zone of differentiation shows two 9+‘1’ axonemes, the elongating nucleus and mitochondrion, and a single cortical cytoplasmic microtubule. This single microtubule is found in the mature spermatozoon of both species and was also noted in capsalids. This requires a modified definition of ‘pattern 2’ of spermatozoa which becomes: ‘spermatozoa with two axonemes and no cortical microtubules, except one single element much shorter than the spermatozoon’. A very unusual structure was found in Myxinidocotyle, but not in Acanthocotyle: the centriolar derivative of one of the 9+‘1’ axonemes is made up of 18 diverging singlets of unequal length associated with electron‐dense cytoplasm. This seems to be the first case of a centriolar derivative without nine‐fold symmetry associated with an axoneme with nine‐fold symmetry.

Lacalli, T. C.

doi: 10.1111/j.1463-6395.1993.tb01229.xpmid: N/A

Abstract A series of laterally projecting ridges develop along the ciliary band of late stage auricularia larvae. These are similar in position to the larval arms of bipinnaria larvae and the epaulettes and vibratile lobes of echinoid pluteus larvae, all of which structures are potentially homologous. When the auricularia is converted to a doliolaria with a series of circumferential ciliary bands, the ridges of the former are retained as basic elements from which the circumferential bands of the latter then develop. There is a simple repeating pattern in the arrangement of these elements in which bands composed of two elements alternate with bands composed of four. The available evidence does not resolve the question of which of the above four larval types, whether feeding or non‐feeding, is more primitive. The common plan apparent among them suggests, however, that this plan, whatever its origin, predates the diversification of larval types among eleutherozoan echinoderms.

Taylor, Paul D.; Jones, Chris G.

doi: 10.1111/j.1463-6395.1993.tb01230.xpmid: N/A

Abstract Scanning electron microscopy of calcified walls in two species of the cyclostome bryozoan Hornera has revealed previously undescribed details of skeletal morphology and growth. The calcitic interior walls of both H. robusta MacGillivray and H. squamosa Hutton have a laminated structure. Walls are extended at distal growing edges where the formation of new crystallites is concentrated and wall fabric is nacreous or semi‐nacreous. New crystallites are seeded on the surface of existing crystallites as six‐sided rhombs. At the centres of the rhombs in H. robusta there are often three ‘spikes' which point towards alternate sides of the rhomb. Screw dislocations resulting in spiral overgrowths are also common at these distal wall edges. Wall thickening occurs further proximally where walls develop a regularly foliated structure of imbricated laths growing towards the colony base. Although often thought to be ubiquitous in cyclostomes, the division of walls into three layers (an inner, primary layer flanked on both sides by secondary layers) is absent in Hornera. Wall ultrastructure contrasts strongly with the lamellar–fibrous–lamellar structure recently described from cinctiporid cyclostomes. The c‐axes of the crystallites are orientated perpendicular to the wall surface in Hornera, unlike cinctiporids in which they are orientated within the plane of the wall. Apparent similarities in ultrastructure suggest that Hornera may provide a good model for wall growth in extinct trepostome bryozoans.

Orrhage, Lars

doi: 10.1111/j.1463-6395.1993.tb01231.xpmid: N/A

Abstract Earlier papers dealing with the microanatomy of the nereid brain have been studied. On this basis a re‐investigation of the cephalic nervous system and of the innervation and homologues of the anterior end appendages of these animals appeared necessary: the existing literature proved insufficient for detailed comparisons with other polychaete families and many earlier statements were quite contradictory. In the present paper, the brain commissures and the innervation of, inter alia, the antennae and the palps of Neanthes virens and Nereis pelagica are described. Special attention was paid to the roots of the circum‐oesophageal connectives and the ganglia in this part of the nervous system. The results, summarized in schematic diagrams and tables, are compared with corresponding observations in 14 other polychaete families. In a discussion of the architecture of the polychaete nervous system as a phylogenetic instrument, the supposed segmentation of the polychaete brain is questioned and the idea that the configuration of the polychaete nervous system offers support to the cyclomer theory is rejected. Other conclusions concerning the relationships within the Polychaeta are pointed out.