At certain seasons, eggs are produced within the body of the parent in appropriate ovaries, where they are retained for a time. They are then transferred to a kind of marsupial pouch, analogous to that of the kangaroo, where their development proceeds. After passing through certain changes here, the egg issues from the maternal pouch as an oval body, clothed with cilia--an animalcule in external aspect, and as unlike its parent as can well be imagined. For awhile the little creature dances freely through the water, and leads a gay, roving life; but at last it prepares to 'settle;' selects a fitting locality; applies one extremity of its body to the surface of stone or weed, and becomes attached. And now another change passes over it. The cilia, no longer needed, disappear. A mouth is developed at the upper extremity of the body, furnished with a number of arms. Gradually this number increases, and the jelly-fish now appears in the disguise of a polype, which feeds voraciously on the members of the class from which it has itself so lately emerged. At this point there is a halt. The medusa remains in its polype state for some months. At the expiration of this term, a strange alteration in its appearance begins to take place. Rings are formed round its body, from ten to fifteen in number. These gradually deepen, until at length it is literally cut up into a number of segments, which rest one upon the other--their upper margins becoming elevated, and divided into eight lobes. It is, in fact, a pile of cup-shaped pieces, very loosely connected together. A little later, these pieces free themselves successively, and the sedate polype disappears in a company of sprightly young medusae. These beings, indeed, still differ in some respects from the adult animal; but the differences gradually vanish, and we have the perfect jelly-fish as the final result of this extraordinary series of transformations. Similar observations have been made respecting other tribes amongst the lower animals, and some interesting generalisations have been founded upon them, into which, however, it is not our present purpose to enter.