Thus of the sexual zoophytes which propagate their kind according to the usual laws; but others engender young which have no resemblance to the parent zoophyte at all: in this respect they are neuter, that is, non-sexual, or _agamous_. These are produced by budding, or fissiparity, from individuals like themselves. They can also give sexual distinctions; but before this change takes place the creature, which was simple, is transformed into a composite animal, and it is from its disaggregation that individuals having sexual organs are produced, the process being that which has been called _alternate_ generation. It goes on in a perfectly regular manner, although it is a fact that the young never resemble their mothers, but their grandmothers. This great family of Zoophytes Gosse divides into-- _Discophora_, having the body in the form of a circular disk, more or less convex and umbrella-shaped, moving by alternate contractions and expansions of the disk. [Illustration: Fig. 84. Æquerea violacea, natural size (Milne Edwards).] _Ctenophora_, body cylindrical, moving by means of many parallel rims of cilia set in longitudinal lines on the surface. _Sophonophora_, body irregular, without central digestive cavity like the others, having sucking organs, and moving by means of a contractile cavity, or by air-vessels. [Illustration: Fig. 85. Aurelia aurita (Lamarck). Cyanea aurita (Cuvier). One-third natural size.] The Discophora are again subdivided into _Gymnophthalmata_, having the eye-specks uncovered or wanting, a great central digestive cavity, circulating vessels proceeding to the margin quite simple or branched; and _Steganophthalmata_ having the eye-specks protected by membranous hoods, or lobed coverings, circulating vessels much ramified, and united with a network. Of the Gymnophthalmata we have an example in _Æquerea violacea_ (Fig. 84), in which the disk is slightly convex, glass-like in appearance, and furnished all round with very short, slender, thread-like, violet-coloured tentacula; with circulating vessels, eight in number, quite simple, and ovaries placed on them; peduncle wide, expanding into many broad and long fringed lobes. The Steganophthalmata include the _Medusadæ_ proper, in which the umbel is hemispherical, with numerous marginal tentacles, eight eyes covered by lobes, four ovaries, four chambers, four fringed arms, with a central and four lateral openings. _Aurelia aurita_ (Fig. 85) is here represented as a type of the group; it is plentiful in the Baltic, and has been carefully studied by the Swedish naturalists. Rosenthal has made its anatomy his special study. Sars has also made it the subject of observations. In the same group we find the _Pelagia cyanella_ of Péron, whose body is globose, scolloped with eight marginal tentacles, peduncles ending in four leaf-like, furbelowed arms, united at the base, having four ovaries, and appendages to the stomach, without orifices.