As the higher animals are nearly always larger than invertebrates, if there be a definite relation between longevity and size, one would expect to find that vertebrates live longer than invertebrates. However, this is not the case. Amongst animals of extremely simple organisation, there are some which reach a great age. A striking example of this is found in sea-anemones. These animals have a very simple structure, without a separate digestive canal, and with a badly developed, diffused nervous system, and yet have lived very long in captivity. More than forty years ago, I remember having seen in the possession of M. Lloyd, the Director of the Aquarium at Hamburg, an anemone that he had kept alive for several dozen years in a glass bowl. Another sea-anemone, belonging to the species _Actinia mesembryanthemum_, is known to have lived 66 years. It was captured in 1828 by Dalyell, a Scottish zoologist, and was then quite adult, and probably about 7 years old. It survived its owner for 36 years, and died in Edinburgh in 1887, the cause of death being unknown. Although they are thus capable of living so long, the rate of growth of members of this species is rapid, and their fertility is very high. According to Dalyell, these anemones reach the adult condition in 15 months. The specimen in his possession, in the 20 years from 1828 to 1848 produced 334 larvæ, then after a period of sterility it gave birth, in one night (1857) to 230 young anemones. This extraordinary prolificness decreased with age, but even when it was 58 years old it used to produce from 5 to 20 at a time. In the seven years from 1872 onwards, it gave birth to 150 young anemones.[35] This animal, which certainly was not more than the fortieth or the fiftieth of the weight of an adult rabbit, lived six or seven times as long. Ashworth and Nelson Annandale have published their observations on another sea-anemone, of the species _Sagartia troglodytes_, which was 50 years old. It differed from younger examples only in being less prolific. There are other polyps, such as _Flabellum_, which do not live more than 24 years, although we have no knowledge as to the cause of the different duration of life. The variation in the length of the life of molluscs and insects is extremely great. Some species of gasteropods (_Vitrina_, _Succinea_) live only a very few years, whilst others (_Natica heros_) can reach thirty years. Some of the marine bivalves, as for instance, _Tridacna gigas_, can live to sixty or a hundred years.[36]