The animals of this order, though of a more complicated external form, have a much simpler internal structure, and much more imperfect vital properties than those of the preceding orders. They possess no external nor internal skeleton, and exhibit no trace of nervous or muscular fibres. They are marine animals of a soft gelatinous transparent texture, and move freely to and fro, supported in the water by the contractility of their fleshy substance, aided in some of the species by vesicles filled with air. There are sometimes se veral entrances to the stomach, but there is no anus distinct from the mouth. They have no dis tinct organs of circulation or respiration, but ves sels or caeca are observed proceeding from the stomach and ramifying through the substance of the body. Many of these animals present beautiful contrasts of colours in their transparent texture, and elegant external plant-like forms. They ap pear to abound chiefly in the colder latitudes of the ocean. They often change the colour of great tracts of the sea from their abundance. They produce the sparkling luminosity often observed by night in the agitated waves of the ocean. They serve as the food of whales, whose masticating organs appear to be adapted for no other kind of food. They present a kind of ovo-gemmiparous generation, and the ova of the Medusa Au vita have been observed to possess a power of spontaneous motion when first separated from the parent, as in Zoophytes. They have no apparent organs for impregnation, nor any distinction of sex. The mouth is in the centre of the lower part of the body, and is generally surrounded with numerous ramified appendices. Many of those animals pos sess a stinging property when touched in the living state with the naked hand, hence the term applied to them by Cuvier, Acalephm (from ,bcci),41 a net tle). The genera of this order are, 1. Beroe. 10. Rhizophysa. 18. Aurelia.
2. Callianira, 11. Sephanomia. 19. Cassiopea.
3. Ceste. 12. Medusa. 20. Lymnorea.
4. Diphia. 13. Equorea. 21. Favonia.
5. Porpita. 14. Foveola. 22. Geryonia.
6. Vilella. 15. Phorcynia. 23. Berenice.
7. Noctiluca. 16. Cyanea. 24. Eudora.
8. Physalia. 17. Rhizostoma. 25. Carybdea.
9. Physsophora.
The animals of this class present a more com plicated and diversified external form, but a more simple internal organization than those of the two preceding classes. They are aquatic animals of a plant-like form, generally compound, and fixed by their base. They possess no distinct organs of sense, nor any trace of a nervous, muscular, or circulating system. Their digestive organs con sist of numerous small superficial sacs termed polypi, having a single aperture, surrounded with tentacula or cilea, for attracting and seizing prey. They have generally a spreading base, a stem and branches, and shoot up from the bottom of the sea, or hang from the roofs of submarine caves, or arc attached to plants, rocks, shells, or other sub merged substances. Their body is for the most part supported by an axis or skeleton composed of cartilaginous, horny, calcareous, or silicious sub stance. The soft parts have a simple gelatinous
texture and soft consistence, destined for the func tion of digestion, and exhibiting no distinct organs either for secretion or generation. Zoophytes are gemmiparous, some generating by the discharge of internal gemmules, and others by forming gem mules on their surface, which separate when they have attained maturity. The internal gemmules have an ovoidal or round form, and soft gelatinous consistence like small ova, and appear, from the observations of Dr. Grant on the zoophytes of the Frith of Forth, to be almost always ciliated on the surface, and capable of moving to and fro in the water, by the rapid vibration of their cilia for some time after their separation from the body of the parent. The small apertures on the surface of Zoophytes for lodging the polypi are called cells, the capsules for containing the gemmules are named vesicles, the arms which surround the mouths of the polypi are the tentacula, and the very minute vibratory processes like short hairs which fringe the tentacula are the cilia. The cells of zoophytes are permanent parts, and their forms afford the most useful characters for distinguishing the species and genera. The vesicles for contain ing the gemmules in the flexible horny zoophytes appear only at particular seasons of the year, and fall off when the gemmules have escaped. In some zoophytes, as the Flustrx and the re productive gemmules are formed in the cells of the polypi, and the polypi and gemmules succeed each other repeatedly during the life of the animal. There are sometimes more than one row of tentacula round the mouths of the polypi, as in Tubularim, Garyophyllix, &c.; most frequently there is but one row, consisting or from 6 to nearly 30 tentacula; 8 is a frequent number in the most highly organized species. In some of the simplest zoophytes the tentacula are entirely wanting. The cilia are generally disposed around the margins of the ten tacula; in most zoophytes they are vibratory, causing, by their motions, currents in the sur rounding water to flow towards the mouths of the polypi. As the fixed condition of zoophytes pre vents them from moving to and fro like other ani mals in search of food, the cilia are adapted to bring it to their mouths from a distance. The cilia are very numerous on each tentaculum, and more with incalculable rapidity. Sometimes more than two thousand cilia are observed on a single polypus, and we have calculated more than three hundred millions of vibratory cilia on a single spe cimen of the Flustra foliacea. When there are no tentacula, the cilia are disposed around the en trance of the digestive sac, as in Vorticelke. Like other gemmiparous animals, zoophytes have no distinction of sex. In many of the genera the gemmules make their appearance in clustres at the base of the polypi, and pass out through their bodies.