_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. [Illustration: Fig. 10. _Mogula oculata._ _a_, branchial; _b_, atrial orifice. ] In the family _Cynthiidæ_ the test is usually leathery, the branchial and atrial apertures four-lobed, and the branchial sac folded into longitudinal pleats. The genera _Boltenia_ and _Culeolus_ include species in which the body is attached to a peduncle. The large exhibited specimen of _Boltenia pachydermatina_ is 28 inches in length, the head being 4 and the stalk 24 inches long. The two four-lobed apertures are along one edge, the branchial being the lower; the body is marked with long deep furrows, and the stalk with transverse wrinkles. _Culeolus perlucidus_, from 1600 fathoms in the Southern Ocean, is in the form of a small pear-shaped head on a slender stalk, the total length being 4½ inches. The branchial orifice forms a transverse slit with raised lips near the stalk, the slit-like atrial orifice being near the rounded end of the body. _Culeolus moseleyi_, another slender-stalked form, was obtained from 2425 fathoms in the Central Pacific. [Illustration: Fig. 11.