OCCURRENCE.—Although for the most part transparent, and usually almost invisible in sea-water, some Appendicularians may have certain parts of the body (alimentary canal, endostyle, gonads, etc.) brilliantly pigmented (orange, violet, etc.), and may under exceptional circumstances be present in such profusion as to colour tracts of the sea. Appendicularians are widely distributed, having been found in all seas from the Arctic to the Antarctic, both round coasts and in the open ocean. Although a few species have been found at considerable depths in the Mediterranean, still in the Atlantic they are not deep-water animals, and as a group must be regarded as surface-forms. They are fairly abundant to a depth of 100 fathoms, and some few reach 1500. Species of _Oikopleura_ and _Fritillaria_ are {70}frequent round the British coasts, our commonest species being probably _O. dioica_, Fol, and _F. furcata_, Moss. Young specimens appear in the plankton about February and March, and larger forms are as a rule found later in the summer. Several instances have been recorded of swarms of especially large forms, provided with massive tests (the "house"), having appeared suddenly on our coast in such abundance as to form an important element in the surface life of the sea. [Illustration: FIG. 32.—Diagram of _Fritillaria_ seen from the right side to show the elongated body, the hood, and the relative positions of anus, atrial opening, and gonads. (Compare with _Oikopleura_, Fig. 30.) _a_, Anus; _at_, opening of atrial tube; _br.s_, branchial sac; _end_, endostyle; _ht_, heart; _m_, mouth; _n.ch_, notochord; _n.g_, nerve-ganglion; _oes_, oesophagus; _ov_, ovary; _sg_, stigma; _sp_, testis; _st_, stomach.] ORDER II. ASCIDIACEA (ASCIDIANS). Fixed or free-swimming Simple or Compound Ascidians, which in the adult are never provided with a locomotory appendage or tail, and have no trace of a notochord. The free-swimming forms are colonies, the Simple Ascidians being always sedentary and usually fixed. The test is permanent and well developed, and becomes organised by the immigration of cells from the body; as a rule it increases in size with the age of the individual. The branchial sac is large and well developed. Its walls are perforated by numerous slits (stigmata) opening into the peribranchial cavity, which communicates with the exterior by the single atrial aperture. Many of the Ascidiacea, both fixed and free, reproduce by gemmation to form colonies, and in most of them the sexually produced embryo develops into a tailed larva. The Ascidiacea includes three groups, the Simple Ascidians, the Compound Ascidians, and the free-swimming colonial _Pyrosoma_, which in some respects connects this Order with the Thaliacea. {71}SUB-ORDER 1. ASCIDIAE SIMPLICES.