. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. A part of one of the arms considerably magnified. In Anatifa, the first pair of arms is thicker and stronger than the others; the sixth pair is the longest. Dr. Grant says, " the arms are not only minutely jointed to their extreme points, but, also, the innumerable fine cilia which pro- ject inwards from their surface are themselves minutely jointed, and by the aid of the micro- scope, we can perceive that these jointed cilia are also ciliated on their ; When the animal is at rest, with the valves of th
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. A part of one of the arms considerably magnified. In Anatifa, the first pair of arms is thicker and stronger than the others; the sixth pair is the longest. Dr. Grant says, " the arms are not only minutely jointed to their extreme points, but, also, the innumerable fine cilia which pro- ject inwards from their surface are themselves minutely jointed, and by the aid of the micro- scope, we can perceive that these jointed cilia are also ciliated on their ; When the animal is at rest, with the valves of the shell closed, the arms are coiled up, and lie close to one another; but, at other times, circumstances being favourable to the perform- ance of the function of respiration, they are ex- tended simultaneously so as to project from the shell,—radiate and plumose in their arrange- ment. Many species extend and contract their arms with considerable rapidity, as often as forty or sixty times in a minute; the smaller species more frequently than the larger. Considering how extensive the surface is which is exposed in the arms between the two rows of cilia, and that a vessel seems to run immediately beneath the delicate covering of these organs in that situation, it appears proba- ble that the arms are very efficient agents in the function of respiration. Secretion.— We have failed to ascertain satis- factorily the structure of the secreting apparatus by which the shells of the Cirripeds are formed. In the Lepads, the organs must be imbedded in the ligamentous membrane by which the valves are united : and in the Balanids, they are arranged in six rows along the outer surface of the mantle, and around the base; but, as in acephalous mollusca, they are too small to ad- mit of their structure being particularly exa- mined. The external surface of the mantle in the Balanids has also the power of secreting calcareous matter, with which to increase the thickness of the shell. Reprodu
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