EMBRYOLOGY OF THALASSEMA MELLITA 217 



long axis. When it first acquires cilia (four and one half 

 hours) it seems indifferent to light, but at about nine hours it 

 shows a slight positive heliotropism. During gastrulation the 

 great majority of the larvae sink to the bottom and do not begin 

 to swim actively until the transition into the trochophore is 

 completed (about eighteen hours). During this time all the 

 energy of the embryo seems to be directed toward the comple- 

 tion of the morphogenic processes. From this time to the 

 seventh or eighth day the trochophores are very decidedly 

 negativ^ely phototactic, but after this they again become indiffer- 

 ent. Strange as it may seem, Mead finds that the embryos of 

 LepidonoUis are affected by light in just the opposite way. 

 When they first begin to swim they are positive, at fourteen 

 hours negative, from twenty to twenty-four hours indifferent 

 and from two to three days strongly positive. Eisig describes 

 the seven-day embryos of Capitella as positively heliotropic, 

 but older larvae as indifferent. 



As has already been indicated, the larvae of TJialassema be- 

 gin to feed as soon as the digestive tract is differentiated and 

 even before the anus has formed. By a gradual loss of cilia, 

 the trochophore sinks to the bottom and undergoes there the 

 metamorphosis into the adult. 



Considered as a whole this trochophore is a very simple and 

 possibly a primitive one. There are no ring-muscles, no nerve- 

 rings and no primary nephridia. Among its positive primitive 

 characters may be reckoned the meager development of the 

 coelomesoblast, the formation of ectomesoblast from the first 

 quartet, the presence of excretory cells, and the late formation 

 of the anus. 



