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The study of comparative anatomy presents the average student with his first detailed view of the structural specialization and functional diversity that are the results of the evolutionary processes. While many aspects are normally covered in lecture and reading assignments, it is personal acquaintance with the material and the opportunity to dissect and study his own animals which make the most lasting impression on the student. There is no substitute for this individual dissection and comparison, and no other activity is likely to yield an understanding of the structures and processes involved.

Comparison poses problems. It is generally impractical to dissect more than a single animal at a given time and only the rare individual is blessed with the gift of looking at any physical structure and remembering its outline and relationships. The majority of individuals achieve this mental image only after they have once transferred the image to paper in the form of a labelled drawing or diagram. The learning function of this process has been repeatedly demonstrated.

Few of us are artists or have had fundamental art training. It is understandable, therefore, that we find it easier to position a structure in relation to another, rather than delimit it on a blank sheet. For this reason, an outline with reference points is desirable. Much time would normally be spent by the student in achieving the proper proportions of such an outline, time that is needed for the analysis of structural detail in the crowded one-semester schedule.

The present atlas attempts to solve these problems for the student of comparative anatomy. In most cases it provides only the outline and reference points of the animal, leaving the details to be supplied by the person carrying out the dissection. This makes it possible to concentrate on the important aspects and aids the student by facilitating immediate recognition, visualization, interpretation and understanding of structure, thus accelerating the learning process.

The atlas covers the spiny dogfish (Squalus acanthias), the mudpuppy (Necturus maculosus) and the cat (Felis domesticus), the three most commonly used and most usefully combined dissection objects for courses of comparative anatomy. The animals dissected and illustrated by us are the actual species available from all U.S. biological supply houses. The dissections of the dogfish thus feature Squalus acanthias, and not Mustellus canis or another rarely encountered form.

All of the drawings are composites of sketches made from actual dissections, and have been checked against photographs of such preparations and where applicable against X-rays as well. The various sheets have been used in actual course work for a number of years, and we are grateful to the many students whose comments and suggestions have been taken into account. The simplicity of the diagrams does not imply lack of care. To the contrary, every reasonable precaution has been taken to assure accuracy and wide applicability.

As visualization of the three-dimensional nature of the material presents both the greatest reward and the greatest difficulty to most students, we have spared no effort to facilitate this. The quality of the drawings emphasizes the third dimension. X-rays and multiple cross sections were used to establish the accurate position of the skeletal elements illustrated within the outline of the intact animal, to make it possible for the student to draw in the muscular insertions rather than to map surface exposures. Cross sectional views present identical structures from more than one aspect.

In addition to the number of customary and almost classical views of dissections, several new views have been worked out and used with considerable success. For example, we show lateral rather than ventral views of the dogfish viscera for presentation of the circulatory system, and sagittal views of the head. In each case the plane chosen allows easier and in use resulted in more accurate interpretation.

While much of the dissection was done by one author (Gans) and all of the drawings by the other (Storr), we feel that this is a truly joint enterprise and the sequence of name implies alphabetical rank alone.