60 



INVEETEBEATA 



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 Olytia,. (After Metsohnikoff. ) 



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

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

 ertrf, 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 

 the typical larva of the Coelen- 

 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 



Fig. 38. — Three stages in growth of fixed planula 

 oiClytia. (After Metsohnikoff.) 



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

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

 which are the rudiments of stolons. 



