248 ECTOPKOCTA. 



what I shall call the anterior end of the oral disc, a structure makes 

 its appearance (fig. 131, m), which is probably homologous with the 

 dorsal organ of the larva of Pedicellina and may go by the same 

 name. It was originally interpreted by Barrois as the pharynx 1 . 



The larva, having now acquired all the important structures it is 

 destined to possess, becomes free. It is shewn in fig. 131 ; the oral 

 face being turned upwards. There are two rings of cilia, one round 

 the edge of the ciliated disc, and a second with larger cilia on the 



FIG. 131. FREE-SWIMMING LARVA or ALCYONIDIUM MYTILI. (After Barrois. ) 

 m (?) dorsal organ ; st. stomodaeum (?) ; s. ciliated disc. 



ring of large cells described above. This ring projects somewhat; 

 its projecting edge being directed towards the ciliated disc. The 

 dorsal organ (mf) is placed on the oral face at the bottom of an 

 elongated groove, in front of which is a bunch of long cilia or flagella. 

 Two long flagella are also developed at the posterior extremity of 

 the oral face, and two pairs (an anterior and a posterior) of eye-spots 

 also appear. Towards the posterior extremity of the oral face is seen 

 a body marked st, which forms the internal sack. If I am right in 

 regarding this as the stomodseum, it is probable that it never unites 

 with the invaginated hypoblast, and that the alimentary tract of the 

 larva remains therefore permanently in an imperfect condition. 



Careful observations have been made by Repiachoff (No. 318) on the 

 early development of Tendra, which accord in some respects with the results 

 arrived at by Barrois in his second memoir. The observations are not, 

 unfortunately, carried down to the complete development of the larva. 



The ovum divides in the normal way into two and then four uniform 

 segments. These four next become divided by an equatorial furrow into four 

 dorsal and four ventral segments, the former constituting the aboral pole 

 and forming the epiblast, and the latter the oral pole. The stages witli 

 sixteen and thirty-two cells appear to be formed in the same manner as 

 in Alcyonidium but between the two layers of cells forming the oral and 

 aboral poles a well-marked segmentation cavity arises at the stage with 

 sixteen segments. At the stage with thirty-two cells the four middle 



1 The interpretation of the larvae given in the text must be regarded as somewhat 

 tentative. The opacity of the free larvae is very great, and almost every one of the 

 numerous authors who have worked on these larva? have arrived at different conclusions, 

 as to the physiological significance of the various parts. 



