688 ZOOLOGY skct. 



their aid of every group in the animal kingdom, and, as a result, 

 our knowledge of the structure of many animals, especially, of 

 the lower forms, of complex organs such as the vertebrate brain, 

 of embryology, and of the minute structure of cells and tissues has 

 been completly revolutionised. Specially remarkable is the advance 

 in our knowledge of the Protozoa, Sponges, Actinozoa, Echino- 

 derms and Amphioxus. The new light which has been thrown on the 

 affinities of Balanoglossus, Ehabdopleura, and Cephalodiscus is also 

 worthy of special mention. Probably the greatest of comparatively 

 recent embryological triumphs, belonging to the earlier part of the 

 period now imder discussion, is Kowalewsky's discovery of the 

 notochord and hollow nervous system of the Tunicate larva, which 

 resulted in the removal of the Urochorda from Molluscoida , to 

 Chordata, and in breaking down the sharp line between Vertebrates 

 and Invertebrates. 



But perhaps the most remarkable result of improved micro- 

 scopical technique is the rise and development of a distinct 

 department of histology, known as cytology, dealing with the 

 minute structure of the protoplasm and nuclei and the various 

 intra-cellular phenomena, such as mitosis. Our knowledge of this 

 subject is entirely a product of the last thirty-five years, and is due 

 in great measure, in the first instance, to the researches of 

 W. Fleming^ E. Strasburger, and E. van Beneden. A 

 inodification of the cell-theory has also been necessitated 

 by the proof that many animal tissues do not consist of dis- 

 tinct 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 it was at one time 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 OdontolciB, 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 pedigrees 

 of the Equidae, Camelidse, and other Mammalian families. Im- 

 portant, though less striking discoveries have also been made 

 among the fossil faunae 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 P. M. 

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



