The dissection of animals is always best performed under water, for by this method the object examined may not only be kept clean as the work proceeds, but the parts, having a tendency to float, readily separate from one another and therefore become more distinctly visible when submerged. [Illustration: FIG. 48.--SHEET OF CORK ON THIN SHEET LEAD] A very convenient form of trough may be made by taking any kind of rectangular, flat-bottomed dish, one made of zinc being, perhaps, the best of all, and covering the bottom with a slab of good cork carpet which has been weighted with sufficient lead to prevent it from floating. Or, instead of cork carpet, a sheet of cork may be used. In either case, a piece of thin sheet lead, a little larger than the slab, should be cut, the corners of which are then snipped off as shown in fig. 48, and the edges finally turned over as represented in the next illustration. The size of the trough must be regulated according to the nature of the work to be done, but one measuring ten inches long, seven wide, and two inches deep will answer most purposes. [Illustration: FIG. 49.--WEIGHTED CORK FOR DISSECTING TROUGH] The object to be dissected is placed in the trough, secured in position by means of a few ordinary pins, and then completely covered with water. We need hardly impress upon the reader the great importance of thoroughly examining all external characters--all those structures that are visible without actual dissection--before attempting to remove anything; and we have already insisted on the importance of carefully examining all creatures while alive before anything else is done. The value of this latter stipulation can hardly be overestimated, for in many instances it is almost impossible to detect the use of an organ unless it has been observed in action; and the enthusiastic student will go even further than this, for he will make it an invariable rule to sketch everything he sees, and to make full notes on all his observations. When pins are used to fix the object under examination--and it is generally essential that the object be fixed--their heads should be turned outwards; for then the object will not slip from its position, nor will the pins tend to get in the way of the work. Some objects are of such a nature that they are not easily secured by means of pins, and yet require to be fixed in some way or other. Thus, one may desire to examine the structure and appendages of a prawn or small crab, or to investigate the nature of a chiton. In such instances as these it is a good plan to make a cake of paraffin wax of suitable size by pouring the melted substance into a mould, then secure the object in proper position in the wax while still fluid, and pin the latter to the cork of the dissecting trough.