45 



The larval tentacles are flattened on the medial side, where 

 the ectoderm is thin. The adult tentacles are circular in section, 

 and the cells are similar in all parts of the section. 



The nervous system of the larva is closely associated with the 

 ectoderm, and lies outside the basement membrane of the ectoderm. 

 It consists of the apical plate ganglion or brain and nerves which 

 proceed from it. Three nerves run forwards, a median and two 

 lateral. The median nerve supplies the papilla of the hood where 

 the nerve expands into another gangHon with a few- nerve cells. 

 Two nerves leave the brain posteriorly and follow the tentacles 

 fusing on the ventral side of the body. In my notes I appear to 

 have observed two pairs of sense organs on the dorsal wall just 

 below^ the level of the septum. 



It has already been stated that the growth of the larva after 

 gastrulation especially takes place by an outgrowth of the post- 

 blastoporal region. The intestine is put into com.munication 

 with the terminal part of this outgrow^th, and in the region inter- 

 vening between the anus and the tentacles a thickening appears. 

 This thickening of the ectoderm is ultimately with the completion 

 of growth carried into the interior as the metasome. As has 

 already been said, it passes in between the two layers of the ventral 

 mesentery, and the mesentery accompanies it as it folds to adapt 

 itself to the space betw^een the body w^all and the alimentary 

 canal. The coelomic epithelium is thickened around the meta- 

 some by developing folds, and the metasome becomes attached 

 in this way also to the stomach. 



The general effects of the metamorphosis are quite apparent. 

 The tube is unrolled as has already been stated, the larval 

 tentacles are thrown off, and after the change has occurred, the 

 tw^o openings of the alimentary canal are rotated towards one 

 another. The oral field is resolved into a horseshoe shape, and 

 the anus is received into the space between the two wings of the 

 horseshoe. The internal changes are also not difficult to appre- 

 ciate. The alimentary canal becomes U-shaped, the trunk 

 cavity of the larva now forms the cavity of the metasome, and the 

 lophophoral cavity takes up its place under the lophopore. The 

 septum serves now therefore to separate a small lophophoral 

 cavity from a large metasomal cavity. 



I am not satisfied even with de Selys Longchamps' evidence 

 that the hood is sacrificed in the process of metamorphosis. At 



