2 ' ZOOLOGICAL CLASSIFICATION. 



seven fundamental forms or types (subkingdoms or phyla) of 

 Animal life. 



They may be tabulated thus : 



Nobody-cavity PROTOZOA. 



A body-cavity [Metazoa]. 

 No backbone. 



No intestinal canal COELENTERATA. 



An intestinal canal. 



More or less of a radiate structure ECHINODERMATA. 



Structure never radiate. 



With legs, or, if without legs, ver- 

 miform. 



Legs never jointed VERMES. 



Legs jointed ARTHROPODA. 



Without legs, never vermiform MOLLUSCA. 



A backbone VERTEBRATA. 



Glaus adds Tunicata, and it has also been proposed that 

 Spongia should rank as a subkingdom. 



The Theory of Descent assumes the common origin of all 

 animals from these seven types ; and that these are derived from 

 a single primaeval form, this, according to Hackel, " originating 

 by spontaneous generation." The Biogenetic law, that the tribal 

 history of the development of organisms (phylogeny) is repre- 

 sented by their individual development (ontogeny), is assumed 

 to be conclusive of the common origin of all animals. 



The primaeval form was a Moneron, and its existence is 

 " attested by the fact that the egg-cell of many animals loses its 

 kernel after becoming fructified, and thus relapses to the lower 

 stage of development of a cytod without a kernel, like a Moneron." 

 This Moneron is individually a Monerula, in the second stage 

 becoming an Amoeba, individually a nucleated ovulum. Syn- 

 amceba, a community of Amoebae, individually a Morula, is the 

 third stage. Planaea, a many-celled primaeval animal without a 

 mouth, its ciliated larva a Planula, marks the fourth stage. 



Lastly we come to Gastraea, another many-celled primaeval 

 animal with intestine and mouth, its larva a Gastrula ; and this 

 was the " common primary form of the six higher animal tribes " 

 (Hackel, Hist. Great., Eng. transl.}. [The modified diagram 

 below shows the pedigree of the Gastraeada, or the animals 

 descended from Gastraea, according to Hackel.] This, the Gas- 

 traea-theory as it is called, has been opposed by Salensky, 

 A. Agassiz, Moquin Tandon, and others. 



