648 ZOOLOGY SECT. 



and E. van Beneden. A modification of the cell-theory has also 

 been necessitated by the proof that many animal tissues do not 

 consist of distinct cells but of a continuous mass of protoplasm 

 with more or less regularly arranged nuclei, and are therefore 

 strictly not multicellular but non- cellular. As certain Protozoa, 

 such as the Mycetozoa and Opalina, are also non-cellular, con- 

 taining numerous nuclei in an undivided mass of protoplasm, the 

 distinction between Protozoa and Metazoa appears to be less 

 absolute than has hitherto been considered. 



The advance in palaeontology during the same period has also 

 been immense. In particular, the researches of E. D. Cope, 

 O. C. Marsh, and others in America have added whole orders to 

 Zoology- -the Odontolca?, Ichthyornithes, Stereornithes, Ambly- 

 poda and Dinocerata and have resulted in the discovery of many 

 new and strange forms among the Dinosauria, Elasmobranchs, 

 Ganoids, and other groups, and in the tracing of the pedigree 

 of the Equida?, Camelidse, and other Mammalian families. Im- 

 portant, though less striking, discoveries have also been made 

 among the fossil fauna? of Europe, India, South Africa, and 

 Australia ; while among Invertebrates the attempts to trace the 

 pedigree of the Ammonites and Brachiopods are specially note- 

 worthy. 



/ 



In embryology an important landmark is furnished by F. M. 

 Balfour's Comparative Embryology (1880-81); in distribution by 

 A. R. Wallace's Geographical Distribution of Animals (1876), each 

 the first complete treatise on the subject in qiiestion. The zoo- 

 geographical regions adopted by Wallace were originally proposed 

 by P. L. Sclater in 1857. Similar landmarks for Zoology as a 

 whole are Huxley's Anatomy of Vcrtebrated Animals (1871) and 

 Anatomy of Invertebrated Animals (1877), Carl Gegenbaur's Ele- 

 ments of Comparative Anatomy (English edition, 1878), Clauses Text- 

 Book of Zoology (1st English edition, 1884-5), Ray Lankester's 

 Notes on Embryology and Classification (1877), and the same author's 

 articles in the Encyclopedia Britannica (9th edition). Both Glaus 

 and Gegenbaur retain Verines as a primary division ; Lankester 

 was the first to split up that unnatural assemblage into distinct 

 phyla, and to include Balanoglossus and the Tunicata among 

 Vertebrates, and Xiphosura and Eurypterida among Arachnida. 

 He also associated Rotifers and Chretopods with Arthropoda, and 

 placed Hirudinea among the Platyhelminthes. 



The student who is interested in the permutations and com- 

 binations of modern classification may be referred to the works just 

 quoted as well as to the numerous text-books published of late 

 years. The most important point to notice in this connection is 

 the breaking down of the sharp boundaries between the four 

 Cuvierian Branches and a return to something like the conception 

 of unity of type, expressed, however, not as a linear series, but as a 



