The structure of the arms of the polypes is very analogous to that of their body. When they are examined by the microscope, either in a contracted or dilated state, their surface is shagreened; if the arm be much contracted, it appears more so than the body; on the contrary, it appears less so in proportion as they are more extended; almost quite smooth when at their full extension; so that in the hydra viridis the appearance of the arms is continually varying, and these variations are more sensible towards the extremity of the arm than at its origin, as, in Plate XXI. Fig. 10; but more thinly scattered, or farther asunder, in the parts further on, as at Fig. 9. The hairs which are exhibited in this figure cannot be seen without a very deep magnifier, however they indicate a further degree of organization in this little animal. The extremity is often terminated by a knob. All animals of this kind have a remarkable attachment to turn towards the light, and this might naturally induce the inquirer to look for their eyes; but how carefully soever this search has been pursued, and however excellent the microscope with which every part has been examined, yet no appearance of this organ has been found. Notwithstanding this, they constantly turn themselves toward the light; so that if that part of the glass in which we placed them be turned from it, they will be found the next day to have removed themselves to the side that is next the light, and the dark side will be quite depopulated. OF THE FOOD OF THE HYDRÆ, AND THEIR METHOD OF SEIZING AND SWALLOWING THEIR PREY. As the hydra fusca, Plate XXI. Fig. 7, has the longest arms, its manner of feeding, and the different manœuvres it makes use of to seize and manage its prey, are more remarkable than those of the two other species; it will be, therefore, this kind only which will be principally spoken of under the present head. To obtain a proper view, it should be placed in a glass seven or eight inches deep. If the polype be fixed near the top of the glass, the arms for the most part hang down toward the bottom. This is a very convenient situation for feeding it, and observing its management of the food. The polypes are in general very voracious: an hungry one extends its arms as a fisherman his nets; it spreads them every way, so that they form a circle of considerable extent, every part of which is entirely within the reach of one of them. In this expanded posture it lies in expectation of its food; whatever comes within the verge of this circle is seized by one or other of its arms: the arms are then contracted till the prey is brought to the mouth, when it is soon devoured. While the arms are contracting and exerting themselves vigorously to counteract the efforts of the animal, which it has seized, to escape, they may be observed to swell like the muscles of the human body when they are in a state of exertion.