In _Botryllus_[103] the oozooid formed from the larva gives rise at a very early period to the first blastozooid of the future colony. This then forms the two buds of the second generation on its sides (see Fig. 55), and these in their turn form the third, and these the fourth generation, in which there are thus eight blastozooids; and so the process goes on, the buds of each generation arranging themselves in a circle to form a system. As each new generation makes its appearance, the preceding one undergoes degeneration, and is eventually absorbed. Consequently, in a system there can usually be seen, in addition to the adult members, certain older ones in various stages of degeneration and removal, and certain younger ones arising as buds on the sides of their predecessors, or just separated from them, and ready to take their places as young ascidiozooids in the system. Three distinct generations are thus commonly seen in a system. Now and again one or two young ascidiozooids become squeezed by the pressure of their neighbours out of a system into the surrounding test, and so give rise to new systems which add to the extent of the colony. [Illustration: FIG. 55.—Diagram to illustrate the budding and formation of a system in _Botryllus_. _Ooz_; oozooid; _Bl_ 1, first blastozooid; 2, 2, etc., successive generations of buds.] SUB-ORDER 3. ASCIDIAE LUCIAE. Free-swimming pelagic colonies having the form of a hollow cylinder closed at one end (Fig. 56). The ascidiozooids forming the colony are imbedded in the common test in such a manner that the branchial apertures open on the outer surface and the {91}atrial apertures on the inner surface next to the central cavity of the colony. They are placed with their ventral surfaces towards the closed end (Fig. 56, C). The first ascidiozooids of a colony are produced by gemmation from a stolonic prolongation of an imperfect oozooid or rudimentary larva (the "cyathozooid"), developed sexually. The subsequent ascidiozooids are formed from these as buds on a ventral stolon. This sub-order includes a single family, the PYROSOMATIDAE, containing one well-marked genus _Pyrosoma_, Péron, with about six species. They are found swimming near the surface of the sea, chiefly in tropical latitudes, and are brilliantly phosphorescent. A fully developed _Pyrosoma_ colony may be from an inch or two to upwards of twelve feet in length. [Illustration: FIG. 56.—_Pyrosoma_. A, lateral view (nat. size); B, end view; C, diagram of longitudinal section. _at_, Atrial apertures; _br_, branchial apertures; _c.cl_, common cloaca; _end_, endostyle; _t_, test; _v_, velum or diaphragm at terminal opening.]