50 THE ANATOMY OF INVERTEBRATED ANIMALS. 



These four common plans are those of the Vertebrata, the M6U 

 lusca, the Articulately and the Hadiata." 



For extent, variety, and exactness of knowledge, Cuvier 

 was, beyond all comparison, the greatest anatomist who has 

 ever lived ; but the absence of two conditions rendered if 

 impossible that his survey of the animal kingdom should be 

 exhaustive, grand and comprehensive as it was. 



Up to the time of Cuvier's death in 1832, microscopic in- 

 vestigation was in its infancy, and hence the great majority 

 of the lowest forms were either unknown or little understood; 

 and it was only in the third decade of the present century 

 that Rathke, Dftllinger, and Von Baer, commenced that won- 

 derful series of exact researches into embryology which Von 

 Baer organized into a special branch of morphology, develop- 

 ing all its most important consequences and raising it to its 

 proper position, as the criterion of morphological theories. 



Upon embryological grounds Von Baer arrived at the 

 same conclusion as Cuvier, that there are four common plans 

 of animal structure. 



In the course of the last half-century the activity of anat- 

 omists and embryologists has been prodigious, and it may 

 be reasonably doubted whether any form of animal life re- 

 mains to be discovered which will not be found to accord 

 with one or other of the common plans now known. But at 

 the same time this increase of knowledge has abolished the 

 broad lines of demarkation which formerly appeared to sepa- 

 rate one common plan from another. 



Even the hiatus between the Vertebrata and the Inver~ 

 tebrata is partly, if not wholly, bridged oyer ; and though 

 among the Invertebrata there is no difficulty in distinguish- 

 ing the more completely differentiated representatives of 

 such types or common plans as those of the Arthropoda, the 

 Annelida, the Mollusca, the Tunieata, the JEehinodermata, 

 the Codenterata, and the Porifera, yet every year brings 

 forth fresh evidence to the effect that, just as the plan of the 

 plant is not absolutely distinct from that of the animal, so 

 1 hat of the Vertebrate has its points of community with that 

 of certain of the Invertebrates; that the Arthropod, the Mol- 

 lusk, and the Echinoderm plans are united by that of the 

 lower worms; and that the plan of the latter is separated by 

 no very great differences from that of the Ccelenterate and 

 that of the Sponge. 



Whatever speculative views may be held or rejected as to 

 the origin of the diversities of animal form, the facts of anat- 



