THE GASTRiEA THEORY AND ITS SUCCESSORS. IO3 



ciliated chambers with their collared cells are respiratory 

 in function. The recent observations of von Lendenfeld, 

 however, demonstrate that the collared cells are inges- 

 tive, and that ingestion takes place in them solely. 

 Even so, however, they are also the cause of the currents 

 of water which pass in through the pores, bringing them 

 food particles, and are therefore also respiratory in func- 

 tion. It seems quite possible that their ingestive func- 

 tion has been secondarily acquired, they being in the 

 best situation for obtaining food. 



As we pass higher in the scale of animal life we find 

 a o-reater and g-reater differentiation of the function of 

 the cells corresponding to the central mass, and we find 

 the endoderm cells assuming more and more a purely 

 digestive function. Except in a few cases the reproduc- 

 tive cells arise from the mesoderm or endoderm (in the 

 Scyphozoa), both of which structures may be considered 

 as derivatives of the central mass of the solid planula. 



Instances having an important bearing upon the idea 

 here proposed are offered by the Orthonectids and 

 Dicyemids. As is well known, in these forms there is 

 a single layer of ectodermal cells enclosing a mass of 

 reproductive cells, or, in the case of the Dicyemids, a 

 single large " endodermal " cell which is the source of 

 the reproductive elements. In development this cell is 

 separated off very early, and is enclosed by the ecto- 

 dermal cells by a process of epibole. The stage preced- 

 ing the complete enclosure of the central cell is generally 

 spoken of as the gastrula stage, but under the present 

 view a Dicyemid or an Orthonectid would be regarded 

 as an adult parenchymella, whose central cells retain 

 their original function, being entirely reproductive. 



