THE COLONY.—The shape of the colony is seen in Fig. 56, A. It tapers slightly towards the closed end, which is rounded. The opening at the opposite end may be reduced in size (see B and C), by the presence of a membranous prolongation of the common test, which can be contracted or expanded by means of the muscle-bands it receives from the atrial siphons of neighbouring zooids. The branchial apertures of the ascidiozooids are mostly placed upon short (in some cases longer) papillae projecting from the general surface, and many of the ascidiozooids have long conical processes of the test extending outwards beyond their branchial {92}apertures (Fig. 57, _t′_). There is only a single layer of adult ascidiozooids in the wall of the _Pyrosoma_ colony, as all the fully developed ascidiozooids are placed with their antero-posterior axes at right angles to the surface and communicate by their atrial apertures with the central cavity (Fig. 56, C). Their dorsal surfaces are turned towards the open end of the colony, and the buds are given off from their ventral edges (Fig. 57). [Illustration: FIG. 57.—Ascidiozooid of _Pyrosoma_ from the right side. _a_, Anus; _At_, atrial aperture; _at.m_, atrial muscles; _Br_, branchial aperture; _br.s_, branchial sac; _cl_, cloaca; _d.l_, dorsal lamina; _d.t_, dorsal tubercle; _ec_, ectoderm; _en_, endoderm; _end_, endostyle; _Ht_, heart; _l.o_, luminous organ; _mes_, mass of mesoderm cells; _m.f_, muscle fibre; _n.g_, nerve-ganglion; _oes_, oesophagus; _sg_, stigmata; _st_, stomach; _stol_, stolon; _t_, test; _t′_, projection of test near branchial aperture; _tes_, testis; _tn_, tentacle; 1, 2, 3, buds.]