60 



INVERTEBKATA 



CHAP. 



oval and ciliated, with an anterior broad and a posterior more pointed 

 end, and which swam freely about in the water. At the pointed 



FIG. 37. Four stages in the development of the planula of Clytiu. (After Metschiiikoff. ) 



A, blastula stage. B, formation of endoderm by immigration of cells of blastula wall at one pole. 

 C, endoderm, a solid mass, half-tilling the cavity of the blastula. D, free-swimming planula larva. 

 end, endoderm. 



end, and at this end alone, cells migrated inwards and formed a mass 

 which rapidly increased in extent, owing, not only to the successive 



immigration of new cells, but 

 also to the division of the im- 

 migrated cells in situ ; and so 

 the blastula was converted into 

 what is termed a planula. 



In the vast majority of 

 Hydrozoa, and also in many 

 Actinozoa, the organism enters 

 on its free life in the " planula " 

 stage. A planula is an oval 

 larva covered with a layer of 

 ciliated cells containing a solid 

 mass of cells inside. Develop- 

 ment within the bell of the 

 parent medusa till the adult 

 form is attained, such as occurs 

 in Tubularia, is exceptional. 

 Hence the planula is termed 



FIG. 38.-Three stages in growth of fixed planula tllG typical larva of the Coeleil- 



of ciytia. (After Metschnikoff. ) terata. This planula, after a 



short free life, attaches itself 

 to the bottom by the broad 

 end, which flattens out. Then, 

 and then only, absorption of the 



central cells takes place, and a gastric cavity makes its appearance. 



The broad end becomes divided by indentations into lobes, each of 



A, at moment of fixation. B, a short time after. 

 C, a day after, st, divisions of broad attached end 

 which are the rudiments of stolons. 



