Studies of the Blood and Tests of Some Australian Ascidians. II. The Test of Pyura stolonifera (Heller)

Abstract

The test of this species is particularly bulky and consists of gelatinous material which encloses the fleshy part of the animal, leaving openings only for the two siphons. It also forms a resilient basal stem which anchors the animal to the substratum. The outer surface of the test is impregnated with sand grains, which form a hard protective layer.

Blood-vessels traverse the gelatinous material. Two large vessels enter the test and these branch to form ultimately a ramifying system of small blood-vessels. Effectively there are two blood circulations in the test. The blood-vessels have a lining consisting of a single layer of large columnar cells, the appearance of which is described.

The gelatinous test resolves itself under the microscope into a network of fibres of variable thickness, but the larger ones at least are aggregates of smaller fibrils. In some cases the fibres are orientated in definite directions. The amorphous interfibrillar material seems to be mainly aqueous, as the test consists of over 97 per cent. of plasma.

Ferrocytes wander about in amoeboid fashion amongst the fibres. These enter the substance of the test by migrating across the walls of the bloodvessels. Histologically there is a marked similarity between the appearance of the test and that of vertebrate connective tissue, as in both cases cells are present in a groundmass enmeshed by fibres which stain with orcein and fuchsin.

However, the fibres of the test are composed of a very insoluble polysaccliaride, Which although resembling plant cellulose in certain respects will not dissolve in special cellulose solvents nor stain with cellulose stains. A mucopolysaccharide which stains metachromatically with toluidine blue is associated with the fibrillar material, and possibly serves to cement the fibres together since these became disarranged on treatment with hyaluronidase, which removes the mucopolysaccharide.

The importance of the test in the general economy of the animal is discussed.