The tornaria larva of hemichordates occupies a central position in phylogenetic discussions on the relationships between Echinodermata, Hemichordata, and Chordata. Dipleurula-type larvae (tornaria and echinoderm larvae) are considered to be primary in the life cycle and thus provide a model for the ancestral animal common to all three taxa (the theory of W. Garstang). If the similarities between tornaria and the larvae in Echinodermata result from homology, their nervous systems should be basically similar as well. The present study utilizes anti-serotonin and FMRFamide antisera together with laser scanning microscopy, and transmission electron microscopy, to describe in detail the nervous system of the tornaria of Balanoglossus proterogonius. Serotonin immunoreactive neurons were found in the apical and esophageal ganglia, and in the stomach epithelium. FMRFamide immunoreactive neurons, probably sensory in nature, were detected in the apical ganglion and in the equatorial region of the stomach epithelium. At the ultrastructural level, the apical organ consists of a columnar epithelium of monociliated cells and includes a pair of symmetrical eyespots. The apical ganglion is located at its base and has a well-developed neuropil. Different types of neurons are described in the apical organ, esophagus, and stomach. Comparison with larvae in Echinodermata shows several significant differences in the way the larval nervous system is organized.This calls into question the homology between tornariae and echinoderm larvae. The possibility of convergence between the two larval types is discussed.

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Spermatogenesis in two species of free-living marine nematodes from the family Chromadoridae (Panduripharynx paci ca and Euchromadora robusta) was studied electron-microscopically. The spermatogonia of both species are undifferentiated polygonal cells with a large nucleus surrounded by a small amount of cytoplasm. In P. paci ca the cytoplasm of spermatocytes contains many Golgi bodies, cisternae of RER, ribosomes, mitochondria and dense spherical bodies. Filamentous material is accumulated in spermatids, which contain only mitochondria and a fragmented (or lobed) nucleus devoid of the nuclear envelope. The immature sperm resembles the late spermatid: its central lamentous area is surrounded by chromatine particles and occasional mitochondria. The immature sperm plasma membrane forms deep infoldings. Mature spermatozoa from the uterus consist of a small main cell body (MCB) bearing a prominent pseudopod lled with cytoskeleton laments. The MCB contains a nucleus and mitochondria. Spermatogenesis in E. robusta (studied only in testes) resembles that described for P. paci ca, but spermatocytes of E. robusta show much lower metabolic activity and, as a result, a smaller mass of lamentous material is stored in the spermatids and immature sperm. The spermatozoa of P. paci ca and the immature sperm of E. robusta have the main ultrastructural features characteristic for nematodes (amoeboid nature, absence of axoneme, acrosome and nuclear envelope). No aberrant organelles special for many nematode sperm (membranous organelles, paracrystalline brous bodies and their complexes) were found during sperm development of the chromadorids studied. In this respect their spermatogenesis differs signi cantly from that in secernents and monhysterids.Résumé -Ultrastructure du développement du sperme chez les nématodes libres de la famille des Chromadoridae (Chromadorida: Chromadorina) -La spermatogenèse a été étudiée en microscopie électronique à transmission chez deux espèces de nématodes libres marins (Panduripharynx paci ca et Euchromadora robusta) de la famille des Chromadoridae. Les spermatogonies, chez les deux espèces, sont des cellules indifférenciées avec un grand noyau entouré d'une petite quantité de cytoplasme. Chez P. paci ca, le cytoplasme des spermatocytes contient de nombreux corps de Golgi, des cisternae du RER, des ribosomes, des mitochondries et des corps sphériques denses. Le matériel lamenteux est accumulé dans les spermatides qui contiennent seulement des mitochondries et un noyau fragmenté (ou lobé) dépourvu d'enveloppe nucléaire. Le sperme immature resemble aux dernières spermatides: son aire centrale lamenteuse est entourée par des particules de chromatine et quelques mitochondries. La membrane plasmatique du sperme immature forme des invaginations profondes. Les spermatozoïdes matures, dans l'utérus, sont constitués par un petit corps cellulaire principal (MCB) portant un pseudopode proéminent rempli de laments de cytosquelette. Le MCB contient un noyau et des mitochondries. La spermatogenèse chez E. robusta (...

Temereva, E.N., Malakhov, V.V. and Yushin, V.V. 2011. Ultrastructural study of oogenesis in Phoronopsis harmeri (Phoronida). -Acta Zoologica (Stockholm) 92: 241-250.The successive stages of oogenesis in Phoronopsis harmeri were examined by electron microscopy methods. During the oogenesis, each oocyte is encircled by vasoperitoneal (coelomic) cells forming a follicle. The previtellogenic oocytes are small cells which accumulate ribosomes for future synthesis; their cytoplasm contains characteristic clusters of mitochondria and osmiophilic particles resembling a germ plasm of other metazoans. The cytoplasm of the vitellogenic oocytes includes numerous mitochondria, cisternae of the rough endoplasmic reticulum, Golgi bodies and annulate lamellae. The synthesis of three types of inclusions was observed: strongly osmiophilic granules (lipid droplets) as a prevalent component, distinctly larger granules surrounded by membrane (proteinaceous yolk) and numerous large vesicles with pale flocculent content. No inclusions which could be unequivocally interpreted as the cortical granules were detected. The surface of the vitellogenic oocytes is covered by microvilli which increase in number and length during development. The oogenesis in Phoronida may be interpreted as follicular because of close association of oocytes with the vasoperitoneal tissue. However, well-developed synthetic apparatus together with a strongly developed microvillous surface and absence of endocytosis indicate a clear case of autosynthetic vitellogenesis. Thus, in phoronids, there is a combination of simply developed follicle and autosynthesis that, apparently, is plesiomorphic character.