ALTERNATION OF GENERATIONS.—Fig. 66 represents an aggregated or sexual _Salpa_, which was once a member of a chain, since it shows a testis and a developing embryo. The ova (always few in number, usually only one) appear at a very early period in the developing chain _Salpa_, while it is still a part of the gemmiparous stolon in the body of the solitary _Salpa_. This gave rise to the view put forward first by Brooks that the ovary {106}really belongs to the solitary stolon-bearing _Salpa_, which is therefore a female producing a series of males by asexual gemmation, and depositing in each of these an ovum, which will afterwards, when fertilised, develop in the body of the male into a solitary or female _Salpa_. This idea, if adopted, would profoundly modify our conception of _Salpa_ as an example of a life-history showing alternation of generations, but it seems to me to give a distorted view of the sequence of events. The fact that the stolon while in the solitary _Salpa_ contains, along with representatives of other important systems of the body, a row of germinal cells, does not constitute that solitary _Salpa_ the parent of the ova which these germinal cells will afterwards become in the body of an independent bud. We must regard as the parent the body in which the ova become mature and fulfil their function. The sexual or chain _Salpa_, although really hermaphrodite in its life-history, is usually[109] protogynous, _i.e._ the ova mature at an earlier period than the male organ or testis. This prevents self-fertilisation. The ovum is presumably fertilised by the spermatozoa of an older _Salpa_ belonging to another chain, and the embryo is far advanced in its development before the testis is formed. The development takes place inside the body of the parent, and is "direct"—no tailed larval form being produced. [Illustration: FIG. 66.—_Salpa hexagona_, Q. and G. Chain form dissected from the left side. _a_, Anus; _at_, atrial aperture; _br_, branchial aperture; _d.l_, dorsal lamina ("gill"); _d.t_, dorsal tubercle; _emb_, embryos; _end_, endostyle; _m.b_ 2, _m.b_ 7, second and seventh muscle-bands; _n.g_, nerve-ganglion; _v_, visceral "nucleus." (After Traustedt.)]