once were accessible only to a handful of swimmers. This has helped to add many to the in- 

 creasing audience for books on invertebrates. Most of the new books that treat at all of in- 

 vertebrates are limited to those of the seashore or of shallow marine waters and deal with the 

 animals from an ecological viewpoint and according to their habitat. This is a much needed 

 approach, and many such books are listed in the bibliography. But the series of which this is the 

 fifth volume (earlier volumes have covered mammals, reptiles, birds and insects, and forth- 

 coming volumes will cover fishes and amphibians) is designed to supply the need for a new set 

 of illustrated natural histories arranged systematically, group by group, and proceeding from 

 the primitive forms to the most specialized ones. Thus the present volume is a natural history of 

 the invertebrates (excepting the insects). But it necessarily presents the animals on a different 

 scale from that of the other volumes, which dealt with no more than a single class of animals. 

 The authors have had to cope with the many invertebrate phyla without allowing the extreme 

 limitations of space for such a project to turn it into a mere catalogue, lacking the vivid detail 

 and discursiveness that make for readability. The plan adopted here seems a reasonable 

 compromise: the smaller phyla are covered only in generalized accounts followed by a treat- 

 ment of a few typical or better-known examples. The large phyla are described in general ac- 

 counts, as are all of their living classes. Below the level of class the treatment is not completely 

 systematic; however, where possible, the specific examples are selected so as to give some 

 representation to all the important orders. Internal structure and embryological evidence are 

 mentioned only when indispensable for understanding of some aspect of behavior or of an 

 animal's position in the evolutionary sequence. For the most part the evidences for classifica- 

 tion are only alluded to: they cannot be adequately expounded in a book of such broad scope. 



It should be noted that the inserts of color plates involve special technical problems and so 

 do not necessarily adjoin the corresponding text, nor do they in all cases follow exactly the 

 sequence of the text. The black-and-white photographs accompany the text and follow the 

 same sequence. 



Natural history is the oldest and the most diffuse of all the branches of biology. A realistic 

 acknowledgement of the written sources of the material in this book would have to begin with 

 Aristotle, who supplemented his own experience by drawing on every possible source, in- 

 cluding fishermen, peasants, and mere hearsay. From his time to ours these same informants 

 have been contributing, along with better-trained or professional observers. This has weighted 

 down natural history with much unreliable information, but it also has given it advantages in a 

 day in which most branches of biology have become so specialized and so experimental as to 

 create an unbridgeable gulf between the professional worker and the interested layman. 

 Although much of the material of natural history is first published in scientific journals, and 

 the authors have drawn mostly on these sources as a matter of habit, the field is one in which 

 original material is also published for the first time in natural history books or even in popular 

 magazines. 



The first half of the text, that covering the protozoans through the entoprocts, was written by 

 Ralph and Mildred Buchsbaum; the second half, that dealing with the chaetognaths through 

 the invertebrate chordates was written by Lorus and Margery Milne. 



