As a general rule the gonophores of Siphonophora do not escape from the parent colony as free-swimming Medusae, but an exception occurs in _Velella_, which produces a number of small free-swimming Medusae formerly described by Gegenbaur under the generic name _Chrysomitra_. This Medusa has a velum, a single tentacle, eight to sixteen radial canals, and it bears the gonads on the short manubrium. The Medusa of _Velella_ has, in fact, the essential characters of the Anthomedusae. Our knowledge of the life-history of the Siphonophora is very incomplete, but there are indications, from scattered observations, that in some genera, at least, it may be very complicated. The fertilised ovum of _Velella_ gives rise to a planula which sinks to the bottom of the sea, and changes into a remarkable larva known as the _Conaria_ larva. This larva was discovered by Woltereck[337] at depths of 600-1000 metres in great numbers. It is very delicate and transparent, but the endoderm is red (the colour so characteristic of animals inhabiting deep water), and it may be regarded as essentially a deep-sea larva. The larva rises to the surface and changes into the form known as the _Ratarula_ larva, which has a simple one-chambered pneumatophore containing a gas, and a rudiment of the sail. In contrast to the _Conaria_, the _Ratarula_ is blue in colour. With the development of the zooids on the under side of this {303}larva (_i.e._ the side opposite to the pneumatophore), a definite octoradial symmetry is shown, there being for some time eight dactylozooids and eight definite folds in the wall of the pneumatophore. This octoradial symmetry, however, is soon lost as the number of folds in the pneumatophore and the number of tentacles increase. It is probable that in the Siphonophora, as in many other Coelenterata, the production of sexual cells by an individual is no sign that its life-history is completed. There may possibly be two or more phases of life in which sexual maturity is reached.