CNIDARIA AS THE ONLY COELENTERATA 25 



The three types of Metazoa, Spongiaria, Cnidaria, and 

 Ctenophora, have not been united into the higher unit of 

 Coelenterata not only for the reason that they lack a secondary 

 body cavity, the coelom (I prefer to call it perigastrocoel) , 

 since there are many other groups of lower Coelomata where 

 the same coelom is absent; but rather because they represent 

 with their general organization a lower stage of development. 

 This, however, does not suffice, as has been demonstrated, 

 to unite them all into one group. The organizations of 

 all these animal groups are different from each other. The 

 thing above all which has to be done if we wish to place 

 them into one common higher systematic category, would 

 be to prove a common origin of all these three groups. 

 This was not necessary in the days of Leuckart. After the 

 general acceptance of the Theory of Evolution the problem 

 was solved first by E. Haeckel, and later by numerous other 

 zoologists (E. Metschnikoff, B. Hatschek, C. Claus, L. v. 

 Graff, the two brothers Hertwig, R. Lankester, A. Balfour, 

 and many other zoologist up to the present day for example, 

 A. Sachwatkin, A. Remane, L. H. Hyman, etc.), in a very simple 

 and quite obvious, way. There was a simply-built primitive 

 form which, though variously called (gastraea according to 

 Haeckel, placula according to v. Graff, parenchymella accor- 

 ding to Metschnikoff, and planula according to Lankester), 

 and differing in details from one another, is, nevertheless, 

 essentially the same. This primitive form has been used 

 in connection with the ontogenetic stages and declared to 

 be the common primitive form of the recapitulation theory 

 as it appears in the "fundamental biogenetic law" proposed 

 by Haeckel. According to this theory three different types of 

 animals have been developed from the one polarized bistratal 

 being which resembled a gastrula. The development diverged 

 in three different directions, and as a result of it we have 

 the Spongiae and Cnidaria which have adopted a sessile way 

 of life, and the Turbellaria which creep over the surface of 

 the sea bottom. The Ctenophora, however, have been usually 



