. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 38 ACALEPH/E. animal's sinking into the deep when danger threatens. In the other physograda, the air-ve- side is so small in pro- portion to the general mass of the animal that it is not sufficient to raise it above the sur- face of the water. It is generally an ovate sac, with an opening at its upper end, closed by a sphincter muscle. It is probable that its walls are muscular,and that by pressing out a portion of the contained air, and by secreting more, alternately the animal can sink and rise at pleasure. The
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 38 ACALEPH/E. animal's sinking into the deep when danger threatens. In the other physograda, the air-ve- side is so small in pro- portion to the general mass of the animal that it is not sufficient to raise it above the sur- face of the water. It is generally an ovate sac, with an opening at its upper end, closed by a sphincter muscle. It is probable that its walls are muscular,and that by pressing out a portion of the contained air, and by secreting more, alternately the animal can sink and rise at pleasure. The nature of the air con- RhlzophW, Melon. !amed in thfe vesicles lias not yet been ascer- tained. In rhizophysa (Fig. 7.) there are, pen- dent from one part of the body, certain peculiar organs, arranged very re- gularly in pairs, of a mus- cular structure, hollow, and furnished each with a round orifice. They differ from the tentacula in structure,and are, pro- bably, organs of natation. Similar tubes, but only two in number, exist in diphysa ; and, anterior -p- Q to them, in the same animal, there is a two-lobed organ, the use of which is doubt- ful. In agalma, (jig- 8.) and some of the genera allied to it, there are certain cartila- ginous plates disposed in an imbricated manner along the sides of the body. These, Eschscholtz regards as loco- motive organs. The mus- cles by which they are set in motion must be extremely delicate, as a slight touch is sufficient to separate the plates from one another. The chief bulk of the sin- gularly formed diphycla is made up of the swimming organs, which are two sub- cartilaginous bodies, poly- gonal, generally pointed an- teriorly, truncated posteri- orly, placed one behind the other, and one a little within the other; the posterior por- tion being lodged in a little excavation which exists in the anterior. These two parts differ somewhat from one another in form, but both are hollow, and have large open- /igahnn nkcnii. ings. Their att
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