OCCURRENCE.—The half-dozen known species of _Pyrosoma_ are widely distributed over the great oceans, although they are probably most abundant in tropical waters. _Pyrosoma atlanticum_, Péron, and _P. giganteum_, Lesueur, are the commonest forms. Although sometimes abundant in the Mediterranean and the North Atlantic they have apparently not been found in British seas. _P. elegans_, Lesueur, is a Mediterranean form allied to the last two; and _P. minatum_ and _P. aherniosum_, Seeliger, were discovered during the German "Plankton" expedition in the tropical Atlantic. Finally, the enormous _P. spinosum_, Herdman, was found by the "Challenger" in both North and South Atlantic in 1873; and {95}some years later (Perrier's _P. excelsior_) by the French "Talisman" expedition in the tropical Atlantic. The late Professor Moseley said of this ("Challenger") species, "I wrote my name with my finger on the surface of the giant _Pyrosoma_ as it lay on deck in a tub at night, and my name came out in a few seconds in letters of fire." Bonnier and Pérez have recently recorded that they saw an enormous profusion of a large _Pyrosoma_ (up to four metres in length) in the Arabian part of the Indian Ocean. ORDER III. THALIACEA (SALPIANS). Free-swimming pelagic forms of moderate size, which may be either simple or compound, and in which the adult is never provided with a tail or notochord. Consequently the whole body here corresponds to the trunk only of the Appendicularian without the tail. The test is permanent, and may be either well developed or very slight. In all cases it is clear and transparent. The musculature of the body-wall is in the form of more or less complete circular bands, by the contraction of which water is ejected from the body, and so locomotion is effected. The branchial sac has either two large, or many small, stigmata, leading to a single peribranchial cavity, into which the anus also opens. Blastogenesis takes place from a ventral, endostylar stolon. Alternation of generations occurs in the life-history, and may be complicated by polymorphism. The Order Thaliacea comprises two groups, CYCLOMYARIA (such as _Doliolum_) and HEMIMYARIA (such as _Salpa_). SUB-ORDER 1. CYCLOMYARIA. Free-swimming pelagic forms which exhibit alternation of generations in their life-history, but never form permanent colonies. The body is cask-shaped, with the branchial and atrial apertures at the opposite ends. The test is moderately well developed, never much thickened. The musculature is mostly in the form of complete circular bands surrounding the body. The branchial sac is fairly large, occupying the anterior half or more of the body. Stigmata are usually present in its posterior part only. The peribranchial cavity is mainly posterior to the branchial sac. The alimentary canal is placed ventrally, close to {96}the posterior end of the branchial sac. Hermaphrodite reproductive organs lie ventrally near the intestine.