There are few groups of the animal world more interesting to the ordinary observer than that of the fishes. To survey the various forms and shapes presented by these animals as displayed in a great museum should prove a sufficient incentive to gain a more intimate acquaintance with the class; and when, even in a popular sense, we investigate the structure and habits of fishes, the study increases in its fascination and interest. While if we reflect that on a knowledge of the habits of fishes, of their distribution in our oceans and seas, and of the special products which many of them offer for our use and luxury, the commercial success of our fisheries depends, it can need no further argument to convince us that, after all, there is something of great practical benefit to be derived from the study of zoological science. It is not our intention at present to say anything regarding the commercial or economic aspects of fishes, and even their general habits must be very briefly touched upon. We rather aim at giving some account of the structure of the fishes, and at noting such peculiarities in their habits and life as may prove most interesting to our readers. Primarily, then, we find that fishes may be recognized by having the body usually, but not always, covered with _scales_, of various forms and kinds. Then, secondly, we have the limbs represented by certain _fins_; and, thirdly, we find almost all fishes to breathe by _gills_ during the whole of life. These three points are, in the main, sufficient to distinguish fishes from their higher as well as their lower neighbors. The scales which cover the bodies of fishes present great diversities in shape, size, and appearance. Some fishes thus exhibit an utter want of scales; while others, like knights of old, are incased in a veritable suit of scaly armor. The lampreys, and their curious neighbors the hag-fishes, are destitute of scales; and in our familiar eels, the scales are very small and insignificant. Such fishes, however, are amply compensated for the want of scales by the power they possess of throwing out from the skin a vast quantity of glutinous or oily matter, technically named _mucus_. The presence of this secretion, which has given origin to the phrase “as slippery as an eel,” serves to protect the surface of the body, and no doubt also assists in the easy progress of these fishes through the water. So large is the quantity of this oily matter which the hag-fishes can emit from their body, that one form has received the specific name of glutinosa; the fish being able in this way to literally convert the water of the vessel in which it is contained into a jelly-like mass. The familiar blennies, found in rock-pools after the tide has receded, are also able to emit a large amount of this glutinous fluid.