SYCETTIDAE Dendy, 1892

Borojevic, Radovan, Boury-Esnault, Nicole & Vacelet, Jean, 2000, A revision of the supraspecific classification of the subclass Calcaronea (Porifera, class Calcarea), Zoosystema 22 (2), pp. 203-263 : 217

Family SYCETTIDAE Dendy, 1892

TYPE GENUS. — Sycetta Haeckel, 1872 by original designation.

DIAGNOSIS. — Leucosoleniida with a central atrial tube and perpendicular regularly arranged radial tubes lined by choanoderm. The distal cones of the radial tubes, which may be decorated with tufts of diactines, are clearly noticeable on the sponge surface. They are never covered by a cortex supported by tangential triactines and/or tetractines. The proximal skeleton of the radial tubes is composed of a row of subatrial triactines and/or tetractines, which are usually followed by only a few or several rows of triactines and/or tetractines. Distal pseudosagittal spicules are absent. A tangential layer of triactines and/or tetractines supports the atrial wall.

DESCRIPTION

In the Leucosoleniida , the transition from homocoel to heterocoel grade of organization apparently passes only through the sycettid-grade of organization (Dendy & Row 1913). The sycettid organization is essentially a sponge like Leucosolenia in which the median region of the single central tube is decorated with regularly arranged short and unbranched radial tubes. It is structurally analogous to the organization of simple Levinellidae from the Calcinea ( Fig. 7 ). The transition from the homocoel to the heterocoel organization involves the progressive restriction of choanocytes to the radial tubes, while the central tube acquires the sole function of an exhalant atrium. In the Sycettidae the elongation of the radial tubes is concurrent with their partial or full longitudinal coalescence around the radial inhalant canals. This organization, typical in the genus Sycon , gives compactness to the sponge, simultaneously maintaining an efficient water circulation.

Figures

FIG. 7. — Diagram of the Sycetta type of organization of the sponge wall. Abbreviations: ps, pinacoderm and the skeletogenous layer; a, atrium; o, osculum. The arrow shows the direction of water flow.