Morphology of invertebrate types

Preface

Owing to the rapid advance of Experimental Zoology, the older branches of morphological study such as Comparative Anatomy and Embryology, have been left in the background. In order to acquire familiarity with modern methods and problems, it became necessary to curtail the time allotted to Morphology. As a natural consequence the laboratory guides in general use in this country are more elementary than is desirable. The result has proved in many ways unsatisfactory. The student is left with only a superficial knowledge of the structure, life and development of those very animals which in his later studies he is going to use for experiments.

But the high tide in Experimental Zoology has been reached and passed. A general tendency to return to morphological studies is manifest in recent years in the great increase in the number of papers dwelling upon the anatomy of invertebrates. It seems, therefore, desirable to place in the hands of the student, a manual which will enable him to lay the foundation for a knowledge of invertebrate anatomy.

The usual method of teaching this subject consists in having the student dissect a series of types, but while doing this he is supposed to pay attention only to structures of a general character and to skip details. This method is open to grave criticism. GeneraHzation should come as the result of extended observation. If we substitute generalization for observation, we do the student's work for him and defeat our own end. By this I do not mean to say that the student should be offered only a knowledge of minute facts and left to his own resources to sift the material and draw conclusions. On the contrary! The

Vi PREFACE

teacher should guide the student until the latter learns to distinguish important from trivial, generalized from specialized, and correlated from disconnected facts. Only in this way will the student learn scientific observation and inductive thought as opposed to mere statement of facts and random conclusions. But in order to accomplish this, the student should first of all learn to observe everything that can be observed from a thorough dissection with simple methods.

Accordingly the treatment of the subject of this manual differs somewhat from the usual. Each chapter consists of two parts: a monograph in which a description is given of the animal selected as representative of its class and instructions for the students to follow in dissection. The descriptions while short, are sufiftciently detailed to include obvious structures of specific value. The monographs are based partly on work done by others, partly on my own dissections and investigations. To these latter are due some of the divergencies from generally accepted opinions as well as statements not to be found elsewhere in print. I have examined all important papers which have been published up to the present time and if I do not give any list of these it is: first, because the student has no need of them; second, because the teacher will know how to get at the sources; and, third, because a comprehensive list would make the book too long and in consequence, too expensive, while a partial list would be of no great value.

The instructions have been followed successfully by my students for two years, and have thus stood the test of the laboratory. The student is expected to read the descriptive part at home, the day before the exercise. The accompanying figures will help him to understand the subject. At the same time he will not be able to copy them in the laboratory instead of making original drawings from the specimen he dissects, for the simple reason that they either represent the structure of some other, though nearly related species, or are diagrammatic. Some of these figures are copies from other books; some are

PREFACE Vli

combination drawings or modified in one respect or another in accordance with my own ideas and dissections.

A few words of explanation are necessary in regard to the choice of material. I tried to use only American species but this proved difiicult in the case of Trematodes and leeches. I have examined many species of Trematodes but none of them is comparable to the common, small, European liver fluke, Dicrocoelium lanceatum. This species does not occur in the Western Hemisphere, but it is a truly typical representative of its class and so common in Europe that it may be bought of foreign firms in unlimited quantities. The medicinal leech is for sale alive in this country and there is therefore no special advantage in giving preference to indigenous species which one would have to collect since they are not on the market. I tried also to avoid species which are usually studied in the Freshman year in the course in General Biology. Experience has shown me that the students resent repetition and surely there is no need for it. As long as the student is required to take General Biology or Elementary Zoology preparatory to Invertebrate Zoology, a diversity of material is of distinct advantage. An unfortunate exception had to be made in the case of the Earthworm, but I do not know of any convenient substitute. A concession to tradition had to be made in the case of the locust. In my opinion a roach is to be preferred, because it is easier to dissect and because it may be kept alive in winter; but private conversation with various colleagues has convinced me that it would not do to leave out the grasshopper. Some groups had to be omitted altogether on account of lack of space. If the book meets with favorable reception, new chapters may be added in a later edition.

The detailed descriptions in the manual of the anatomy of animals which the student is given to dissect, will relieve the teacher of the necessity of lecturing on this subject. His time may be better spent in giving a general account of the group to which each animal belongs and in treating broader aspects viii PREFACE

of the problems involved in the study of Morphology. The whole aim of the teacher should be to attain a perfect balance between laboratory work and lectures, for the one teaches methods, the other, results.

Alexander Petrunkevitch.

New Haven, Conn.

May, 1916.