The spermatozoa are generally matured and shed before the ova, and thus self-fertilisation is prevented. The ova are very small, and little is known of the development. [Illustration: FIG. 30.—Longitudinal optical section of _Oikopleura_. Part of the tail is cut off. _a_, Anus; _at_, atrial opening; _br.s_, branchial sac; _c.f_, ciliated funnel; _ec_, ectoderm; _end_, endostyle; _ep.p_, epipharyngeal ridge; _g.d_, opening of gonads to exterior; _ht_, heart; _hy.p_, hypopharyngeal ridge; _i_, intestine; _m_, mouth; _mus_, muscle-bands in tail; _n_, nerve-cord; _n′_, nerve in tail; _n.ch_, urochord; _n.g_, nerve-ganglion; _n.g′_, ganglion in tail; _oes_, oesophagus; _or.gl_, oral gland; _ot_, otocyst; _ov_, ovary; _sg_, stigmata; _so_, sense-organ; _sp_, testis; _st_, stomach; _t_, test. (After Herdman.)] CLASSIFICATION.—There are two Families of Larvacea: First, the KOWALEVSKIIDAE, including only the remarkable genus _Kowalevskia_, Fol, in which the heart and endostyle are absent, and the branchial sac is provided with four rows of ciliated tooth-like processes. The two known species have been found in the Mediterranean and in the Atlantic. The second family APPENDICULARIIDAE comprises about eight genera, amongst which may be mentioned:—(1) _Oikopleura_, Mertens, and (2) _Appendicularia_, Fol, in both of which the body is short (1 or 2 mm. in length) and compact (Fig. 30), and the tail relatively long, while the endostyle is straight. (3) _Megalocercus_, Chun, from deep water in the Mediterranean; _M. abyssorum_ is the largest Appendicularian known, having a total length of {69}3 cm.—it is of a bright red colour. (4) _Fritillaria_, Q. and G., in which the body is elongated (Fig. 32) and composed of anterior and posterior regions, the tail relatively short, the endostyle recurved, the stigmata opening far in front of the anus, and an ectodermal hood is formed over the front of the body. In all nearly forty species of Larvacea are known. [Illustration: FIG. 31.—Transverse section of body and tail of _Oikopleura flabellum_ (?) _at_, Atrial tube; _bl.s_, blood-space; _br.s_, cavity of pharynx or branchial sac; _ec_, ectoderm; _en_, endoderm; _ep.p_, epipharyngeal ciliated bands; _gel_, gelatinous layer between ectoderm and endoderm; _hy.p_, hypopharyngeal ciliated band; _mus_, muscular tissue on inner surface of ectoderm of tail; _n_, nerve-cord; _n′_, its continuation in the tail; _n.ch_, notochord in tail; _r_, rectum; _sg_, one of the stigmata or ciliated openings from the branchial sac to the atrial tube; _t_, test (= young "house"); _x_, bridge of gelatinous tissue in front of stigma closing branchial sac off from atrial tube. (After Herdman.)]