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Among tunicates, gamete morphology and sperm-egg interactions have been extensively investigated in ascidians, and to a lesser extent in appendicularians and thaliaceans. Sperm-egg interaction has been studied in only one salp, Pegea socia (Bosc, 1802). To determine if the pattern of internal fertilization of P. socia is generally applicable to salps, we performed an ultrastructural study on blastozooids of Thalia democratica (Forsskal, 1775). The ovary, located in the mantle near the gut, consists of a single oocyte connected to the atrial chamber wall by a 'fertilization duct', resembling a stack of single cells without a lumen. The flagellate sperm has a long corkscrew-like head with the single mitochondrion twisted around the nucleus. Fertilization is internal, and sperm actively penetrate the atrial wall and bore through the cells of the fertilization duct. During this process, the fertilization duct shortens as the cells move apart, one to one side and the next to the other, and rejoin to form a central lumen, which contains many sperm. At the same time a few sperm reach the periovular space for fertilizing the oocyte. Comparisons with P. socia indicate that this singular mode of internal fertilization with a complex corkscrew sperm actively penetrating the fertilization duct cells, probably evolved in the salp ancestor and has been modified to some extent in various genera. Chez les tuniciers, la morphologie des gametes et les interactions spermatozoides-oeufs ont ete bien etudiees chez les ascidies, mais moins bien chez les appendiculaires et les thaliaces. Les interactions entre les spermatozoides et les oeufs sont connues chez une seule salpe, Pegea socia (Bosc, 1802). Afin de determiner si le patron de fecondation interne de P. socia s'applique de facon generale aux salpes, nous avons etudie l'ultrastructure des blastozooides de Thalia democratica (Forsskal, 1775). L'ovaire, situe dans le manteau pres du tube digestif, consiste en un seul oocyte relie au mur de la chambre de l'atrium par un << conduit de fecondation >> ressemblant a un amoncellement de cellules individuelles sans lumiere. Le spermatozoide flagelle possede une longue tete en tire-bouchon avec une seule mitochondrie tournee autour du noyau. La fecondation est interne et les spermatozoides penetrent activement dans la paroi de l'atrium et percent les cellules du conduit de fecondation. Durant ce processus, le conduit de fecondation raccourcit, alors que les cellules se detachent, les unes d'un cote, les autres de l'autre, et se rejoignent pour former une lumiere centrale qui contient de nombreux spermatozoides. Au meme moment, quelques spermatozoides atteignent l'espace periovulaire pour feconder l'oocyte. Les comparaisons avec P. socia indiquent que ce mode particulier de fecondation interne, dans lequel un spermatozoide complexe en tire-bouchon penetre activement dans les cellules du conduit de fecondation, a probablement evolue chez l'ancetre des salpes et s'est modifie jusqu'a un certain degre dans les differents genres. [Traduit par la Redaction], Introduction Tunicates are marine animals and include benthic ascidians, planktonic thaliaceans (salps, doliolids and pyrosomes), and appendicularians. Most are hermaphroditic and have highly varied modes of sexual reproduction that ensure [...]

Abstract: The distribution of sensory cells and nerves was studied in the siphons of Corella inflata Huntsman, 1912 and Corella willmeriana Herdman, 1898 by immunohistology and electron microscopy. Each siphon [...]

The morphology and some functions of the gut of Fritillaria pellucida and Fritillaria formica (Fritillaridae) were investigated by light and electron microscopy, and also by means of histochemical and immunohistochemical techniques. Fritillarids, very important for their abundance and ecological impact in marine ecosystems, have a very simplified gut: a straight oesophagus connects the pharynx to the digestive nucleus, composed of globular stomach and rectum, connected dorsally through a very short proximal intestine. The latter is characterised by a few (two to four) extremely specialised cells, completely filled with mitochondria associated tightly with membrane infoldings showing strong ATPase activity, and probably involved in the osmoregulation of internal body fluids. The gut is formed of an extremely low number of cells, which, although poorly diversified, are very large in the stomach and rectum. Food transfer along the gut depends on and is regulated by well-developed cardiac and pyloric valves, and signs of general digestive and absorptive activity are recognisable all along the brush border of the main tracts. The macroscopic organisation and cytological characters of the gut in fritillarids are completely different from those of the oikopleurids. In particular, fritillarids lack specialised cells for endocytosis and intracellular digestion, like those described in the genus Oikopleura. The general simplification and specialisation observed in Fritillaria gut may account for their elevated growth rate and abundant diffusion in all oceans.

The functions of the various gut regions of Oikopleura dioica (oesophagus, left and right gastric lobes, vertical intestine, mid-intestine and rectum) were investigated by means of histochemical, histoenzymatic and immunohistochemical techniques at light and electron microscopes. Ciliary food progression is evidenced by the presence of ATPases on cilia, along the entire gut, with the exception of the cardiac valve, a passive device controlling food direction. Absorptive processes (alkaline phosphatase), active transport (ATPases) and nitrogen excretion (D-amino acid oxidase) occur along the entire gut, in both ciliated microvillar and globular cells. The latter, typical of the left gastric lobe and rectum, are also involved in endocytotic processes (exogenous peroxidase as tracer) and intracellular digestion ([alpha]-amylase, aminopeptidase M, acid phosphatase, 5'-nucleotidase, non-specific esterase). The giant cells of the gastric band participate in extracellular digestion; they contain secretory granules positive to various hydrolytic enzymes, the activity of which is also recognisable in faecal pellets inside the intestinal lumen. Lipid storage occurs mainly in the right gastric lobe and vertical intestine, whereas protein storage takes place in the rectal granular cells. Epithelial transport and possible osmoregulation occur along the entire gut, especially at the level of diffuse baso-lateral interdigitations, which increase the plasmalemma surface enormously, are often associated with mitochondria and possess numerous ATPase pumps. Data extend previous histological observations and hypotheses on the physiological role of the various gut regions. The remarkable and specific location of enzymatic activities and nutrient storage are in agreement with the high capacity of O. dioica to process a great quantity of food very rapidly and efficiently.

The pathway of spermatozoa to approach the oocyte in the sexual reproduction of the colonial axidian Diplosoma listerianum is studied histologically and ultrastructurally. Internal fertilization involves many adaptations including the female fertilization canal origin, tight injunctions and complex sperm morphology.

Ultrastructural evidence proves the morphological relationship between the ovary, egg envelopes and oocyte in the sexual reproduction of axidian Diplosoma listerianum. Sperm-egg interaction involves an internal fertilization and the sperm penetrates through ovary wall preceded by a step of metamorphosis. Phylogenetic aspects of internal fertilization are discussed.