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.
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.
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.
Fig. 18.
Colonies of Ascidiæ compositæ, natural size. A. Colella quoyi. B. Leptoclinum neglectum. C. Pharyngodictyon mirabile. D. Botryllus.
(After Herdman, Challenger Report and Encyclopædia Britannica.)