Although reduced to an extremely small size each individual or ascidiozooid of a colony possesses the same organs as a large Simple Ascidian, excepting that the former does not possess a separate test. Frequently the individuals of a colony are grouped into systems, in which the atrial orifices open into a common cloaca. The little ascidiozooids vary greatly in shape in the different families. In the _Polyclinidæ_, for instance, they are long, the organs being so to speak, drawn out, and being arranged in three regions, the thoracic, abdominal and post-abdominal, the first region containing the branchial sac, the second the stomach, and the third the heart and reproductive organs. In the _Distomidæ_, the body exhibits two regions, thoracic and abdominal, the heart and reproductive organs lying alongside of the stomach. The _Botryllidæ_ comprise only one region, the stomach and the other organs being situated by the side of the branchial sac. The Compound Ascidians include seven families which are characterised chiefly by the method of bud formation, and by the arrangement of the organs into one, two, or three regions. It is only possible, from limits of space, to refer to a few interesting forms. The species of _Botryllus_ are those most commonly met with. They form richly coloured gelatinous incrustations on rocks and seaweeds. _B. violaceus_ (Figs. 16, 17, and 18 D) is blue with white lines; _B. smaragdus_, green; _B. marionis_, brown with white and carmine; _B. castaneus_, purple, and so on. The individuals are arranged in circular systems with the branchial orifices round the circumference and the atrial orifices opening into a common central cavity (Fig. 17), the whole colony being composed of groups of systems. [Illustration: Fig. 16. _Botryllus violaceus_ on seaweed. (After H. Milne-Edwards.) ] The exhibited specimen of _B. violaceus_ was grown in the tanks of the Biological Station at Plymouth. The red specimen of _B. aurolineatus_, from Naples, shows well the branchial and cloacal orifices. In _Botrylloides_, the individuals form elliptical or elongated systems. _Colella thomsoni_ was obtained near the Philippines at a depth of 10 fathoms. The specimen, which is about 7 inches in length, resembles an elongated head of clover on a thickened stalk. The individuals which compose the head are arranged in spiral lines, the atrial orifice of each ascidiozooid opening separately and not into a common cloaca. [Illustration: Fig. 17. A. _Botryllus violaceus_, magnified, showing two systems of 6 and 7 ascidiozooids. B. One ascidiozooid extracted. _a_, branchial; _b_, atrial orifices; _c_, branchial sac; _d_, stomach. (After H. Milne-Edwards.) ] _Colella quoyi_ (Fig. 18 A), from 25 fathoms off Kerguelen Island, forms a rounded head on a short peduncle, the total height being one inch. The ascidiozooids are arranged in vertical lines in the “head,” each line consisting of a double zigzag series. [Illustration: