[Illustration: FIG. 28.—Appendiculariida. A, _Appendicularia sicula_, Fol, with house; B, _Megalocercus abyssorum_, Chun, nat. size; C, _Oikopleura cophocerca_, Gegenb., with house; D, _Fritillaria megachile_, Fol, with vesicle; E, Appendicularian in its house; F and G, two stages in the formation of the house. (A to D from Seeliger; E to G from Lohmann.)] The anterior (cerebral) ganglion has connected with it an otocyst (Fig. 29), a pigment spot, and a tubular richly ciliated process opening into the branchial sac, and representing the dorsal tubercle and associated parts of an ordinary Ascidian. The tube ends in a plain or coiled cellular mass lying to the right of the ganglion. No neural gland is found. {67}[Illustration: FIG. 29.—Transverse section through anterior part of _Oikopleura_ to show ganglion, sense-organs, endostyle, etc. × 300. _br.s_, Branchial sac; _c.f_, ciliated funnel; _ec_, dorsal ectoderm; _end_, closed anterior end of endostyle; _hy_, hypobranchial groove in floor of branchial sac; _n.g_, nerve-ganglion; _or.gl_, oral gland; _ot_, otocyst; _x_, opening of ciliated funnel into pharynx.] The branchial aperture or mouth leads into the simple branchial sac or pharynx (Fig. 30, _br.s_). There are no tentacles. The endostyle is short, is a closed tube both anteriorly and posteriorly (Fig. 29), and has about four longitudinal rows of gland-cells. There is no dorsal lamina, and the peripharyngeal bands run dorsally and posteriorly to unite close in front of the oesophageal opening. The wall of the branchial sac does not show the complex structure usual in Tunicata, and has only two ciliated apertures (Figs. 30, 31, 32, _sg_). These are homologous with the primary stigmata of the typical Ascidians, and with a pair of the gill-clefts of Vertebrates. They are placed far back on the ventral surface, one on each side of the middle line, and lead into short funnel-shaped tubes which open on the surface of the body behind the anus (Fig. 30, _at_). These tubes correspond to the right and left atrial involutions, which in an ordinary Ascidian fuse to form the peribranchial cavity. The remainder of the alimentary canal consists of oesophagus, stomach (which may have a glandular diverticulum), intestine and rectum (Fig. 30). The heart, surrounded ventrally by a delicate pericardial membrane, lies below and in front of the stomach, and is formed by the differentiation of the outer ends of epithelial cells into muscular fibrillae. Two specially large glandular cells are placed at the opposite ends of the heart. There are no blood-vessels except the remains of the primary body-cavity (blastocoel). No heart can be seen in some of the smaller species of _Oikopleura_. Nearly all the species are hermaphrodite, and the large ovary and testis are placed at the posterior end of the body. There is no proper oviduct, the genital {68}products merely breaking through to the exterior at the point marked _g.d_ in Fig. 30.