8 INTRODUCTION. 



The " micromillimeter " is not an original member of the Metric System, and the 

 desirability of using the word is strongly questioned. In either case, the symbol for the 

 thousandth of the millimeter should be the Greek /z rather than mmm. 



15. The Metric System in Medicine. Notwithstanding the 

 obvious and considerable hindrances to a change involving commercial inter- 

 ests, the new system is surely though slowly making its way among Physicians 

 and Pharmacists. 



The matter is constantly discussed in the Medical Journals, and " Reduction Tables " 

 have been published in very compact form. Dr. Kreider has one, in the Medical Record 

 for Oct. 23, 1880 ; Dr. F. H. Brown has another in " The Medical Register for New Eng- 

 land," which has been reprinted as a pocket leaflet by the Metric Bureau ; while another 

 leaflet, first printed by the Metric Bureau (A, 345-348), by Dr. E. Wigglesworth, presents 

 " The Metric System in a Nut-shell," with especial reference to the needs of the 

 medical profession. 



ZOOLOGICAL CLASSIFICATION. 



16. As this is in no sense a treatise upon either Zoology or Com- 

 parative Anatomy, but simply a guide to certain practical methods of work, 

 we give only such an outline of the Classification of Animals as may serve to 

 indicate the generally accepted taxonomic relations of the forms here con- 

 sidered. 



Fuller information and discussion of this matter may be found in the works and 

 papers of Gegenbaur, Haeckel, Huxley, Owen, etc., and in the condensed Summary of 

 Pascoe (A) ; see also Balfour, A, ii, 1. 



It did not seem worth while, in the accompanying Table, to indicate the view enter- 

 tained by many Zoologists, that the primary division of both the Animal Kingdom and 

 the Vertebrate Branch is dichotomom : the one into the Protozoa and the Metazoa ; the 

 other into the Acrania and the Craniota. 



The names of the eight animals more or less fully treated of are given in 

 the column at the right of the page. One, the amceba, is a member of the 

 lowest division of the Animal Kingdom, the Protozoa, and consists of but a 

 .single cell, a mass of nearly homogeneous but nucleated protoplasm. 



The other seven belong to the highest division, the branch Vertebrata, 

 the essential character of which, as will be more fully indicated farther on, 

 is that there is a dorsal and a ventral cavity, between which is a sub-cylin- 

 drical axis of membrane, cartilage, or bone. 



Man, the Cat, Dog, and Rabbit are members of the class Mammalia, 

 including the Vertebrates which are warm-blooded, and are brought forth 

 alive. The Frog and Menobranchus are Amphibia or Batrachians, which 

 differ from the Reptiles in having gills or water-breathing organs at some 

 period of their lives. 



