Fixed Ascidians, which are solitary, and very rarely reproduce by gemmation; if, as in a few cases, small colonies are formed, the members are not buried in a common investing mass, but each has a distinct test of its own. No strict line of demarcation can be drawn between the Simple and Compound Ascidians; and one of the families of the former group, the Clavelinidae (the "Social" Ascidians of Milne-Edwards), forms a transition from the typical Simple forms which never reproduce by gemmation, to the Compound forms which always do. Over 500 species of Ascidiae Simplices are now known, but there are probably very many more still undescribed. The sub-order may be divided into the following families:— FAM. 1. CLAVELINIDAE.—Simple Ascidians which reproduce by gemmation to form small colonies (Fig. 33), in which each member, or ascidiozooid, has a distinct test, but all are connected by a common blood-system, and by a prolongation of the "epicardiac tubes" (see p. 83) from the branchial sac. Buds are formed on the stolons (Fig. 33), which are vascular outgrowths from the posterior end of the body, containing prolongations from the ectoderm, mesoderm, and endoderm (the epicardium) of the Ascidiozooid. Branchial sac not folded; internal longitudinal bars usually absent; stigmata straight; tentacles simple. The Clavelinidae are the simplest of the Ascidiae Simplices. They are the forms that come nearest to the Compound Ascidians, and are closely related to the Distomatidae. They are probably the nearest representatives now existing of the ancestral forms from which both Simple and Compound Ascidians are descended. [Illustration: FIG. 33.—Colony of _Clavelina lepadiformis_ (nat. size).] This family contains amongst others the following three genera:—_Ecteinascidia_, Herdman, with internal longitudinal bars in the branchial sac; _Clavelina_, Savigny, with a long body and intestine extending behind the branchial sac (Fig. 33); and {72}_Perophora_, Wiegmann, with a short compact body and intestine alongside the branchial sac. _Clavelina lepadiformis_ and _Perophora listeri_ are common British species found at a few fathoms depth off various parts of our coast. Both occur round the south end of the Isle of Man. In autumn _Clavelina_ accumulates reserve-material in the ectoderm cells of parts of the stolon, which remain when the rest of the colony dies away, and then form new buds in spring. FAM. 2. ASCIDIIDAE.—Solitary fixed Ascidians, never forming colonies; with gelatinous or cartilaginous test; branchial aperture usually eight-lobed, atrial aperture usually six-lobed; branchial sac not folded; internal longitudinal bars usually present; stigmata straight or curved; tentacles simple; gonads in or around the intestinal loop. This family is divided into three sections:— SUB-FAM. 1. HYPOBYTHIINAE.—Branchial sac with no internal longitudinal bars, test strengthened with curious symmetrically placed nodules.