256 Prof. E. Hackel on the Position of the 



advantage, in order to express this tlioroughgoing homology, 

 to designate the primordial rudiment of the intestine, such as 

 persists through life in the simplest form in Olynthus and 

 Hydray as i\\Q, pi'imitive intestine (JJr da nn ^ p'og aster) ^ and its 

 orifice as the primitive mouth [Urmund, prostoma) , especially 

 as, according to Kowalevsky's statements, this primordial 

 mouth-opening appears (at least in many animals) to represent 

 not the future permanent mouth, but the future anus. 



The true hody-cavity^ which is usually termed the pleuro- 

 2^eritoneal cavity in the Vertebrata, and for which we propose 

 instead of this sesquipedalian term the more convenient de- 

 nomination coeloma {KolXcofia, a cavity), occurs only among 

 the higher animal stocks, the Vermes, Mollusca, Echino- 

 dermata, Arthropoda, and Vertebrata. As the ontogeny of 

 the Vertebrata shows us, this coeloma always originates be- 

 tween the inner and outer germ-lamell£e, by a splitting of the 

 middle germ-lamella into a cutaneous and an intestinal fibro- 

 lamella. Now, as the middle germ -lamella is entirely deficient 

 in the Sponges, no coeloma can occur in them. It is equally 

 absent in the Acalephce, although in these a middle germ- 

 lamella (mesoderm, or muscular lamella) is already developed. 

 It is therefore of great importance to our monophyletic theory 

 of descent that t/ie lowest Vermes (Turbellaria, Trematoda, 

 Cestoda, &c,) are also entirely destitute of a coeloma, which is 

 only developed in the higher Vermes {Vermes coelomati), from 

 which it has been inherited by all the four higher stocks. 

 The Vermes without a body-cavity ( Vermes acoelomi) are in 

 this respect " Coelenterata.'''' 



The true body-cavity, or coeloma, therefore, can never, like 

 the intestinal or stomachal cavity, be enclosed by the ento- 

 derm. Leuckart certainly says expressly (even in 1869), 

 " The body-cavity of the Coelenterata is not situated between 

 the exoderm and entoderm, but is enclosed by the latter;" 

 but this very statement proves that Leuckart's conception of 

 the " Coelenterate type " is quite erroneous. Neither can the 

 body-cavity ever communicate directly with the stomachal 

 cavity or the intestinal cavity, as is said to be the case with 

 the Coelenterata in the writings of Leuckart and many other 

 authors. The anatomy and ontogeny of the coeloma, or pleuro- 

 peritoneal cavity, in all the higher animals shows rather that 

 this true hody -cavity is from the first commeyicement a perfectly 

 distinct cavity, quite independent of the intestinal tube, which 

 is never connected with it. The buccal opening never leads 

 into the true body-cavity; and when Leuckart and others 

 conceive of the intestinal or stomachal cavity of the Coelen- 

 terata as a " body-cavity," they ought, to be consistent. 



