Cystid Structure and Protrusion of the Polypide inCrisia(Bryozoa, Cyclostomata)

Abstract

Nielsen, C., Pedersen, K . J . 1979. Cystid structure and protrusion of the polypide in Crisia (Bryozoa, Cyclostoinata) . (Marine Biological Laboratory, Helsingar and Institute of General Zoology, Copenhagen, Denmark.) -Acta 2001. (Stockh.) 6 0 ( 2 ) : 6 5 4 8 .T h e ultrastructure of the cystid of Crisia eburnea has been studied. The cystid wall comprises an outer periostracum, a calcified layer and one inner cell layer, the ectoderm. The membranous sac, which consists of an outer basement membrane, a series of very thin annular muscle cells and an inner layer of epithelial cells, is interpreted as the detached mesoderm of the cystid wall. Accordingly, the atrial sphincter and the generally eight branched, longitudinal muscle cells connecting the terminal membrane and the membranous sac are interpreted as ectoderm. The membranous sac is attached to the cystid wall with two lateral ligaments and at four abfrontal areas: 1) a wide distal area, 2) an area at the origin of the retractor muscles, 3 ) a small area between 1 and 2, and 4) a small basal area at the origin of a pair of small muscle cells attached to the lowermost part of the caecum.We infer that the protrusion of the polypide is caused by a sequential contraction of the annular muscle cells of the membranous sac, starting basally and aided by the contraction of the longitudinal ectodermal muscle cells.

Animal phylogeny in the light of the trochaea theory

“…The structure of the shells is very similar to that of the calcified exoskeletons of many gastroneuralians (see, for example, Nielsen & Pedersen, 1979), but the terebratulids also have a calcified endoskeleton (in the lophophore) comparable to that of the echinoderms (Schumann, 1973).…”

Skeletal ultrastructure and phylogeny of cyclostome bryozoans

“…The latter possess internodes linked by nodes or joints which are elastic and permit some degree of flexing of the colony. Joints apparently form by the progressive resorption from within of the calcareous skeleton (Borg, 1926: 259) to be replaced by an annulus of elastic material (Nielsen & Pedersen, 1979). (0) These patterns relate at least in part to interactions between the feeding currents of individual zooids to produce colonial patterns of water flow (see McKinney, 1990).…”

“…This conclusion is founded on several characters regarded as synapomorphies which are known to be present in disparate recent cyclostomes but are not found in the living outgroup (Ctenostomata). Cyclostome synapomorphies are: (1) polyembryony, whereby the primary embryo divides repeatedly to produce clonal secondary and tertiary embryos (Harmer, 1893); (2) the membranous sac, a muscular internal membrane enclosing an inner body cavity (entosaccal coelom), interpreted as a detached peritoneum and functioning in tentacle protrusion (Nielsen & Pedersen, 1979); (3) lack of or rudimentary frontal cilia on the tentacles (Nielsen, 1987;Riisggrd & Manriquez, 1997;Nielsen & Riisggrd, 1998); (4) a poorly-developed funicular system which does not link to communication pores in the walls between zooids; (5) larval brooding in enlarged polymorphic zooids (gonozooids) or apparently derivative 'zoarial brood chambers' (see Silkn, 1977b;Schafer, 199 1) (polymorphic brooding zooids appear to have been secondarily lost in the Cinctiporidae, see p. 54); (6) absence of a larval apical organ (Nielsen, 1995: 202); (7) larval metamorphosis to give a calcified, dome-shaped protoecium, otherwise referred to as the primary disk (Nielsen, 1970); (8) a calcitic adult skeleton with a predominantly lamellar fabric; (9) interior skeletal walls containing pores lined by inward growing spines.…”

Skeletal Ultrastructure in Some Articulate Cyclostome Bryozoans

“…The cuticle then sinks inwards before an annulus of elastic material (Nielsen & Pedersen 1979) develops in place of the calcified wall, bursting the cuticle as it expands. Borg did not study Crisulipora and it is not known whether the same sequence occurs in this genus.…”

“…Among the works dealing with the soft part anatomy of articulates, those by Nielsen (1970) on larval metamorphosis, and by Nielsen and Pedersen ( 1979) on cystid structure are particularly notable. All zooidal polymorphs in articulates are fixed-walled, i.e.…”