The star-fishes have distinct sexes, with individual differences; their eggs, which are round and reddish, undergo curious phases of development. They produce little worm-like creatures, covered with vibratile hairs, like the infusoria, which swim about with great vivacity; these little creatures are subject to considerable changes. In the year 1835 M. Sars described, under the name of _Bipinnaria asterigera_, an enigmatical animal resembling a polyp from the arms at one extremity of the body, while the other terminated in a tail, furnished with two fins; but it was chiefly remarkable as having an Asterias attached to the extremity which carried the arm. He expressed an opinion, which was soon placed beyond any doubt, that this _bipinnaria_ was an Asterias in its course of development. The egg becomes a sort of infusoria, the infusoria becomes a _bipinnaria_, and this produces the Asterias. In short, the Bipinnaria does not become an Asterias by any metamorphoses analogous to that so well known amongst insects--the butterfly, for example--but becomes, so to speak, the foster-mother or nurse to the Bipinnaria. The larva is large, and it is at the cost of a very small internal rudiment of this larva that the Asterias is developed: the Asterias robs the larva of its stomach and intestines, and turns it into a visceral apparatus for its own use. But the Asterias makes itself a mouth of any of the pieces most remote from the primitive mouth of the larva. Thus the Bipinnaria divides itself; it gives its stomach and intestines, and keeps its oesophagus and mouth, and it can live several days after the Asterias is detached from it.