THE LARVA OP ASTERIAS VULGARIS. 113 



portiou, thus forming a single continuous bilaterally 

 symmetrical ciliated band, by Semon named the circum- 

 oral band. The course of this band is shown in figs. 12 and 

 14, c. 0. b. Whether the two legs of the band touch, fuse, or 

 remain separated at the apex of the preoral lobe is somewhat 

 difficult to determine. Semon in Asterias rubens finds 

 that they touch; in Asterias vulgaris I am inclined to 

 believe that they are separated (see figs. 23 and 25). 



At the apex of the preoral lobe (fig. 15) the ectoderm-cells 

 are elongated over a small area. The outer part of the cell is 

 clearer with fine granules, while at the base of the cells are 

 what appear to be the cut ends of fibres, suggesting nerve- 

 fibres. This may possibly be regarded as a very simple apical 

 plate. It is the same as that referred to above in case of the 

 gastrula (fig. 9). The direction of locomotion has become 

 reversed. At first the blastopore was directed forwards; now 

 the blastopore has become the larval anus, and is near the 

 posterior end. The reversal of direction in locomotion takes 

 place at the time of the formation of the mouth. 



Ad oral Band. — At first the entire surface of the larval 

 oesophagus is ciliated. The first trace of that ciliated band 

 immediately surrounding the mouth and sending a loop into 

 the cEsophagus, called by Semon the adoral band, is a thick- 

 ened circular ring of ectoderm closely surrounding the mouth 

 opening (fig. 12, a. o. b.). Later, with the formation of the 

 depression of the body-wall around the mouth the circum- 

 ference of this ring increases, and is seen to surround the rim 

 of this oral depression (fig. 17, a. o. b.). It is to be noticed 

 that up to this time there is no connection whatever between 

 the circumoral and this adoral ciliated band. But later the 

 transverse preoral portion of the circumoral band becomes 

 pushed towards the hinder end of the larva, and finally vaults 

 over the oral depression, and almost covers the mouth (figs. 24 

 and 30, c. o. b.). In this way the anterior part of the adoral 

 band comes to touch and fuse with the posterior transverse 

 preoral portion of the circumoral band. Thus the connection 

 between these two bands is secondary, and not primary as was 



