| Order I. Ascidiacea. | Sub-order 1. | Ascidiæ | Simplices, 4 Families. |
| Sub-order 2. | „ | Compositæ, 7 Families. | |
| Sub-order 3. | „ | Salpiformes, 1 Family. | |
| Order II. Thaliacea | 3 Families. | ||
| Order III. Larvacea | 1 Family. | ||
Order I.—Ascidiacea.
The Ascidiacea include the great majority of species. With the exception of the one genus Pyrosoma, they lead a fixed or stationary life.
Sub-order 1.—Ascidiæ Simplices.
The Simple Ascidians are mostly solitary; in a few forms, however, colonies arise by budding from stolons, but each individual has a distinct test. The four families into which the sub-order is divided are chiefly characterised by the nature of the test, the number of lobes round the branchial and atrial orifice, and the character of the branchial sac.
In the family Molgulidæ the tough membranous test is often coated with sand; the branchial aperture is six-lobed, the atrial four-lobed, the branchial sac has long folds or pleats, and the stigmata are curved or arranged in spirals.
Molgula gigantea, which is one of the largest of the Ascidians, and which attains a length of over thirteen inches, forms a tough conical sac; the branchial and atrial orifices at the upper end have six and four lobes respectively. The test is leathery, smooth above, but coated with sand below. The exhibited specimen, which comes from the Straits of Magellan, has several specimens of the stalked Boltenia legumen attached to the lower part of the test.
The curious Molgula oculata (Fig. 10) has a soft oval or rounded body coated with sand. The branchial and atrial orifices have respectively six and four lobes. Specimens grow attached to the rocks and also live free in the sand. The surface of the test is provided with hairs, which adhere to the rocks and collect particles of sand. The adhesion not being very firm, specimens are easily detached by currents and collected into heaps by the eddies; when living in the sand only the two dark orifices are visible. The sand coating has been supposed to confer protection by mimicry of the environment; but Professor Lacaze Duthiers found, much to his chagrin, that the sandy tests of his specimens were of no avail in securing them from being devoured by crabs who seemed to scent their prey from afar.
Fig. 10.
Mogula oculata.
a, branchial; b, atrial orifice.