SMITHSONIAN MISCELLANEOUS COLLECTKJNS 



VOL. 



The body of the planula is usually larger at the anterior end 

 (figs. 2,3), and only in this does the planula attain cephalization in 

 the Strict sense. Its principal structural dififerentiation occurs at the 

 posterior pole, where there takes place an ingrowth of cells 

 (fig. 2 B-D) that soon fills the hollow of the body, and finally, by the 

 appearance of a cavity within its mass, becomes the wall of the stomach 

 of the mature animal. The process of forming a primitive stomach, or 

 archenteroii. as it takes place in the planula, is typified by that of 

 gastrulation in ordinary embryonic development (fig. i A-D). The 

 planula, of course, is a specialized larval form, and its manner of 

 cndoderm formation can not be taken as showing how the archenteron 

 was evolved, but the free-swimming planula does show that the primi- 

 tive mouth, or hlasfoporc (fig. jC,D,Bp), was formed at the 



A 



B 



C 



D 



Fig. 2. — Formation of the endoderm in a coeleiiterate planula larva by pro- 

 liferation of cells from posterior pole. (From Hatschek, 1888, after Claus.) 



Blc, blastocoele ; Pld, blastoderm ; Bed, ectoderm ; End, endoderm. 



posterior pole of the body, and not at the anterior pole. It is interest- 

 ing to note, therefore, that the position of the mouth opening was 

 not necessarily a primary determining factor of cephalization ; the 

 practical site for a mouth in a free-swimming, ciliate animal was 

 determined by the direction of the animal's movement. Korschelt 

 and Heider (1895) have stated, if a monaxial, heteropolar planula is 

 allowed to swim through water containing particles of carmine, it 

 can be seen that the particles are rej^ulsed at the anterior and lateral 

 parts of the body, but that they accumulate at the posterior pole. 

 '' Here accordingly," say Korschelt and Heider, " was a favorable 

 place for the reception of particles of food, and by a flattening or 

 shallow invagination of the posterior pole these favorable conditions 

 were increased. The archenteron, therefore, in its earliest beginnings 

 was a pit in which to catch particles of food." 



