Order PHLEBOBRANCHIA

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Order PHLEBOBRANCHIA

Compiler and date details

P. Kott, Queensland Museum, Brisbane, Queensland, Australia

Introduction

The suborder Phlebobranchia (order Enterogona) is characterised by having unpaired gonads present only on the same side of the body as the gut. As in Stolidobranchia, the body is not divided into different sections (such as thorax, abdomen and posterior abdomen) as the gut is folded up in the parietal body wall outside the pharynx and the large branchial sac occupies the whole length of the body. Usually the branchial sac (which is flat, without folds) has internal longitudinal vessels (although only vestiges remain in Agneziidae). Epicardial sacs do not persist in adults as they do in Aplousobranchia, although excretory vesicles (nephrocytes) embedded in the body wall over the gut are known to originate from the embryonic epicardium in Ascidiidae and Corellidae. Most phlebobranchs are solitary. However, Plurellidae Kott, 1973 includes both solitary and colonial forms, and Perophoridae Giard, 1872 are all colonial. Replication in Perophoridae is from ectodermal epithelium (rather than endodermal or mesodermal tissue the mesodermal tissue of the vascular stolon (rather than the endodermal tissue as in most as in Aplousobranchia). The process of replication has not been investigated in Plurellidae.

Phlebobranch taxa occurring in Australia are documented in Kott (1985). Family level taxa are characterised principally by the size and form of the branchial sac including the number of branchial vessels and form of the stigmata; the form, size and position of the gonads; and the habit (colonial or solitary) of the taxon. Berrill (1950) has discussed problems in assessing the phylogeny of Perophoridae.