Thus the whole family is divisible into two groups, the _simple_ and the _aggregate_; both branching out into numerous genera, of which my limits only allow me to mention some of the most remarkable. The simple Ascidiæ, or Sea-Squirts, are very common on our shores. "Rarely," says Forbes, "is the dredge drawn up from any sea-bed at all prolific in submarine creatures without containing few or many of their irregularly shaped leathery bodies, fixed to sea-weed, rock, or shell, by one extremity, or by one side, free at the other, and presenting two more or less prominent orifices, from which, on the slightest pressure, the sea-water is ejected with great force. On the sea-shore, when the tide is out, we find similar bodies attached to the under surface of rough stones. They are variously, often splendidly, coloured, but otherwise are unattractive or even repulsive in aspect. Some are of a large size, several inches in length. As may easily be imagined, they lead a very inactive life, except in the young state, when by means of a long tail they rapidly swim about, until finally settling in some convenient spot, they gradually assume the form and adopt the quiet life of the parent from which they sprang." [Illustration: Chelyosoma Macleayanum. _a._ Branchial orifice. _b._ Anal orifice. _c._ Coriaceous envelope of the sides. _d._ Stone to which the animal is fixed. ] To the simple Tunicata belong also the Chelyosomata, whose coriaceous envelope, consisting of eight somewhat horny angular plates, reminds one of the carapace of the turtle. Their small and prominent orifices, perforating the plated surface, are each surrounded by six triangular valvules. Some species of simple Ascidians on the coasts of the Channel and the Mediterranean are valued as articles of food. At Cette sea-squirts are taken regularly to market, and _Cynthia microcosmus_, although so repulsive externally, furnishes a very delicate morsel. [Illustration: Botryllus violaceus. Two of the stems magnified. _a._ Common test. _b._ Some of the branchial orifices. _c._ The common anal orifice of one of the systems. ] [Illustration: Diazona violacea (magnified).]