POLYZOA. 299 



surface, which gives rise to a sack opening to the exterior 

 (fig. 131, j/.). This was originally held by Barrois to be the 

 stomach ; but Barrois now prefers to call it ' the internal sack.' 

 To my mind it is probably the stomodseum. The embryo has 

 become in the meantime laterally compressed, and, at what 

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

 its appearance (fig. 131, ;;/), 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 



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

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



cilia, one round the edge of the ciliated disc, and a second with 

 larger cilia on the ring of large cells described above. This 

 ring projects somewhat ; its projecting edge being directed 

 towards the ciliated disc. The dorsal organ (m?) 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 



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 larvae have arrived at different conclu- 

 sions, as to the physiological significance of the various parts. 



