142 Marvels of Pond-Life. 



classification of peculiarities that might have been 

 deemed of the highest importance. 



Professor Huxley divides the vertebrates into Ithy- 

 coiDS, comprising fishes and amphibia, which, besides 

 other characteristics, have gills at some period of their ex- 

 istence; Saurotds (reptiles and birds), which have no 

 gills, and possess certain developmental characteristics in 

 common ; and, lastly, Mammals. The Insecta, Myrio- 

 poda, Arachnidse, and Crustacea, he remarks, " without 

 doubt present so many characters in common as to 

 form a very natural assemblage. All are provided 

 with articulated limbs attached to a segmented body 

 skeleton, the latter, like the skeleton of the limbs, 

 being an ' exoskeleton,' or a bordering of that layer 

 which corresponds with the outer part of the verte- 

 brates. In others, at any rate in the embryonic con- 

 dition, the nervous system is composed of a double 

 chain of ganglia, united by longitudinal commissures, 

 and the gullet passed between two of these commissures. 

 No one of the members of these four classes is known 

 to possess vibratile cilia. The great majority of these 

 animals have a distinct heart, provided with valvular 

 apertures, which are in communication with a peri- 

 visceral cavity containing corpusculated blood.^"* These 

 four classes have constituted the larger group or 

 *^ province'' of Articulata or Arthropoda. Professor 

 Huxley thinks that, notwithstanding " the marked dif- 

 ferences'' between the Annelida (worms) and the pre- 

 ceding Arthropods (joint-foots), their resemblances out- 

 weighing them — '' the characters of the nervous system, 

 and the frequently segmented body, with imperfect 



