CLAVELINIDX, a family of Tunicated ifollusca, including the British genera Clavelina and Perophora. This family may be regarded as uniting together the Compound and Simple Ascidians. Till very recently it was supposed that the animals forming this family belonged to the latter. Milne-Edwards first pointed out that the animals which had been described by Savigny under the genus Clavelina were not always, nor even usually, separated from each other ; but that they spring, as it were, from a common creeping stem, and multiply by gemmatiou in the same manner as the Compound Ascidians. Milne Edwards also pointed out that an animal, described by Mr. Joseph Jackson Lister iu the 'Philosophical Transactions' for 1834, WM truly au Ascidian. This animal occurs in groups consisting of several individuals, each having its own heart, respiration, and system of nutrition, but fixed on a peduncle that branches from a common creeping stem. The individual animals were connected together by a circulation extending throughout the stem. They are transparent, so that their structure can easily be seen through their membranous covering. Milne-Edwards proposed for these animals the name of Social Ascidians.
Clavelina, Savigny.—The individuals and groups are connected by creeping radiciform prolongations, the animals having elongated erect more or less pedunculated bodies. The branchial and anal orifices without rays; outer tunic smooth and transparent ; thorax usually marked with coloured lines.
C, lepadiformis (Ascidia lepadiformis, 0. F. Muller). Thorax forming a third part of the length of the adult individual, and marked with yellow lines ; stomach of a bright orange, placed near the middle of the abdominal portion of the animal ; part of the intestines of the same colour. Mr. Alder says this animal is very generally diffused throughout the coasts of Great Britain. He has met with it on tho Devonshire, Cornish, and Northumberland coasts, and in Lamlash, Rothsay, and Oban bays in Scotland. Mr. W. Thompson has found it in Ireland.
Milne-Edwards gives the following account of- the development of this species :—" If we examine with care the foot of a C. lepacliformis,.
we see that the animal adheres to the soil by more or less numerous radiciform prolongations of the tegumentary tunic ' • and usually we find also cylindrical filaments, which mingled with these roots and formed externally by the same tissue, creep also on the surface of the soil ; but are hollow, and internally furnished with a membranous tube. This tube is ooutinuous with the internal tunic of the Ascidian, and the circulation which is seen in the interior of the abdomen of the latter is equally continued into the appendicular canal. This stalk-like body, which is closed at the extremity, is at first simple, but ramifies as it elongates. When its growth is more advanced,
we see developing at the extremities of its branches, or even at different points of its length, tubercles containing in their interior a little organised mass in connection with the internal tube. Theso tubercles elongate, elevate themselves vertically, and become clavi form ; the blood which circulates in the stem penetrates the soft and pyriform central mass ; but this mass, at first peduncujated and adhering to the inner tunic of the principal canal, soon separates itself, and no longer participates in the circulation of the individual to which it owed its origin. Nevertheless its development continues, and we soon distinguish in it all the principal characteristic traits of the ascidian structure; the branchial sac becomes perfectly outlined without being as yet in communication with the interior ; a curved digestive tube is seen beneath the thorax. At length a buccal opening is formed, and the general shape of the young animal approaches more and more nearly that of the adult. Thus there is produced by process of budding a new individual, linked with its parent by a radi ciform prolongation of the tegumentary tunic, and which during the first years of its life has a circulation in common with the mother ascidian, but in the end enjoys an independent existence. Still how ever it may remain in connection with the individual which produced it through the medium of its roots, or it may become completely free by their rupture without any change of consequence in its mode of life." M4moires de l'Institut,' vol. xviii.) There are several other species of Clavelina, and probably many more exist on our own coasts.
Perophora.—The animal discovered by Lister has been thus named by Wiegmann. It is characterised by the individual animals being pedunculated, suborbicular, compressed, attached by their pedicles to creeping tubular processes of the common tunic, through which the blood circulates. Thorax not lineated by granular bands.
P. Listeri is the only species at present known. It is a minute creature, and occurs not unfrequently on the south coasts of England and in the Irish Sea. It lives attached to sea-weeds, end is beautifully transparent. It looks to the naked eye like little specks of jelly dotted with orange and brown, and linked by a silvery winding thread. Mr.
Lister's paper describing this animal is entitled,' On the Structure and Functions of Tubular and Cellular Polypi, and of Ascidise: It is an admirable paper, and was one of the first-fruits of those labours on the improvement Of the microscope for which the world is indebted to Mr. Lister.