ABSTRACT
The Bryozoa are triploblastic animals, in which the mesoderm proceeds from an embryonic endomesoderm, topographically distinct in the young embryo from the ectoderm. Generally, the endodermal cells degenerate before the end of embryogenesis, but sometimes origin to a functional larval digestive tract in some specialized and presumed primitive
Cheilostomes (Malacostegans sensu stricto) or an abortive structure with various destinies according the evolutionary lineages (d’Hondt 1976). Coelomogenesis in the Eurystomes (it is unknown in the Stenolaemates) occurs not during the embryogeny but only during metamorphosis (d’Hondt 1974, 1982), and later a part the ancestrular coelom is transmitted at each budding to a new zooecia by the ancestrula, and further from each mother-zooecia to her daughter-zooecia. A reinterpretation of the old observations on the
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Jean-Loup d’Hondt Muséum National d’Histoire Naturelle, “Biodiversité des Communautés Aquatiques”, Département “Milieux et Peuplements Aquatiques” (Biologie des Invertébrés Marins), Paris, France
ABSTRACT: The following revised definition of the phylum Bryozoa is given after a critical discussion of their main biological characters in the light of recent advances and controversies in our knowledge of this taxon. ‘Colonial animals which are morphologically and anatomically unsegmented, triploblastic, with embryonic endomesoderm and generally abortive endodermic macromeres (excepted some primitive and specialized Cheilostomes); endocoelomates without archimery, in both larva and adult; two-layered – ectodermic and mesodermic – tegument, each layer with peculiar morphological and ontogenetical capacities, in both the larva and the adult; according to the cases, chitinous or chitino-calcareous exoskeleton; embryologically with protostomian larva, but atypical deuterostomian ancestrula (and consequently it is also the case for the functional autozooecia); neither hyponeurian nor epineurian in the adult stages; some epidermal cell lineages, true stem cells, retaining a permanent totipotence from the larval metamorphosis to the death of the zoarium, the latter proceeding by clonal development from a single larva; presenting, according to the different phylogenetic trends, various types of ontogenetic capacities of morphogenetical substitutions; capacities of epidermic cells are polarized; each neopolypidial morphogenesis is an predetermined biological phenomenon, probably under hormonal influence; devoid of some physiological systems (respiration, circulation, excretion); adult constituted by an visceral component, the polypide, and a tegument, the cystid; with a row of peribuccal tentacles in the adult, and of ectodermal origin; ectoprocts; generally with an intermittent digestive system (main part of the polypide), periodically degenerating and renewed from an epidermic proliferation to the inside (consequently: 1° – the adult epidermis has kept some morphogenetic capacities of a gastrula; 2° – the budding – by disappearing of an inhibition factor – from an ectodermis, of an organ with a digestive vocation, contradicts the classical theory of embryological layers); digestive system U-shaped, without digestive gland; without coelom in the larva, the coelomic cavity appearing by schizocoelie only during the metamorphosis. Some evolutionary trends genetically “programed” and predetermined, characterized by embryological apoptosis and pre-differentiations, events obvious from the embryological stages. Elaborate coloniality (interzoidal pore and funiculesrosettes system); presenting capacities of both asexual and sexual reproduction and a very complicated metamorphosis. The species are mainly sedentary and benthic, exceptionally free-living; always aquatic, generally marine; filter-feedings with a predominantly vegetarian diet. They often display an interzooecial polymorphism corresponding to a functional specialization of individuals’.
