The edges of the orifices are more or less crenulate or divided. The branchial orifice is 8-9 rayed, and the anal 6 rayed, in Phal lusia ; both orifices are 4-fid in Cynthia, Bol tenia, and Dendrodoa. When contracted, they are thrown into longitudinal folds ; this is the more observable when the orifices are somewhat prolonged into tubes.
The test receives from the body blood vessels, which its semitransparency in some species allows the eye to follow to the extreme ramifications. In the thinner tests the vessels are but few, and sometimes altogether escape observation ; but in the thick pellucid test of Ascidia mammillata the eye can discern an extensive network of vascular ramifications. The bloodvessels enter the test near the base. The internal surface of the test has often a glistening and pearly appearance, and is always lined with an epithelium. In Boltenia reni. fonnis this is a loose tissue, and forms a slight attachment to the external surface of the mus cular sac or mantle of the body.
In A. mammillata a thin vertical septum traverses a part of the cavity of the test.
The test of the Clavellinidce is very similar in consistence and general appearance to that of some of the more delicate species of the Ascidiadce, but differs materially in having tu bular prolongations running from its base, which are traversed by vessels continuous from one individual to another, and from which root-like processes young individuals are continually being produced by the process of gemmiparous reproduction (fig. 768.).
The tube-like test of the Salpidwis "semicar tilaginous, or gelatinous, seeming as if carved in crystal," its transparency rendering conspi cuous the brilliant hues of the liver of the contained animal. It is usually more or less angular, bearing elongated crests, denticles, and other processes, by means of which the cohesion of the aggregated individuals is maintained (fig. 772.).
The tests of the little animals of' which the cylindrical body of the Pyrosoma is composed, are intimately connected by their lateral sur faces, leaving their terminal orifices free (fig. 786. a). The tissue of the whole common envelope is seinicartilaginous, transparent, tinted with azure and other colours.
In the BotryllichE the external tunic is re presented by the tegumentary tissue com mon to the whole association of animals, and which may perhaps be compared to the polypary of aggregated Polyps. The close aggregation of the animals causes a fusion, as it were, of the tests of the whole into a coriaceous or gelatinous mass, coated with a tough epidermic membrane, and more or less regularly beset with individuals or groups of individuals (systems) ; or, rather, the soft test of the originally isolated indivi dual, or single system, increasing in bulk and producing within itself more and more embryos of individuals and systems, becomes an ex tended, homogeneous mass, in which nu merous individuals are lodged (fig. 771.). This
mass is irregular, fungous, jelly-like, spongy, or coriaceous, incrusting other sessile Tuni cates and a variety of marine bodies.
Structure and chemical composition of the test. —We are indebted to the labours of MM. Lii wig and Kolliker * for considerable information on the subject of the constituent elements and the growth of the test of the simple and the compound Ascidians. These researches were undertaken with the view of examining into Dr. Schmidt's statement of the existence of the vegetable element " cellulose" in the tissue of the ascidian envelope. This they found to be correct (as the following state ments will show), and they have also offered a lucid explanation of the fact, to which we shall subsequently refer. Cellulose forms the cell walls of vegetables, and is unaffected either by soda or hydrochloric acid.
The cartilaginous envelope of Phallusia mammillaris (Fam. Ascidiadce), examined in specimens preserved in spirit, is composed of three layers of different thicknesses (fig. 773.). The internal layer, formed simply of polygonal, nucleated, epithelial cells, measuring 0•00,5"', covers all the interior surface of the test ; at the two external orifices, and at the points where it receives the nutrient vessels of the test, it is united with another epithelial tissue covering the mantle. The second layer is con siderably thicker, and is composed of a homo geneous substance, containing crystals and nuclei. The former are not present every where, and are, perhaps, quite absent in the recent animals ; when present they are visible to the naked eye, and appear like white strim ; seen under a moderately magnifying power, they have the form of crystals united in the form of a star, or of irregular and polymor phous concretions. The nuclei are present in considerable numbers and under different forms ; those situated towards the interior are round, 0•0015"'-0002"', with one or two opaqtie nucleoli, similar to fat-granules. The external nuclei are larger, round or more or less produced, and contain clear or granular substance, and usually some opaque granules.