GEPHYREA. 



357 



Larval forms and development of organs. 



Amongst the Gephyrea armata the larva has as a rule 

 (Thalassema, Echiurus) the characters of a trochosphere, and 

 closely approaches the typical form characteristic of the larva of 

 Polygordius, often known as Loven's larva. In Bonellia this 

 larval form is less perfectly preserved. 



Echiurus. In Echiurus (Salensky, No. 368) the youngest 

 known larva has all the typical trochosphere characters (fig. 161). 

 It is covered with cilia and divided into a prae-oral lobe and 

 post-oral region of nearly equal dimensions. There is a double 

 ciliated ring which separates the two sections of the body as in 

 the larva of Polygordius : the mouth (;) opens between its two 

 elements. The alimentary canal is divided into a stomodaeum 

 with a ventral opening, a large stomach, and a short intestine 

 opening by a terminal anus (an). Connecting the oesophagus 

 with the apex of the prae-oral lobe is the usual contractile band, 

 and at the insertion of this band is a thickening of the epiblast 

 which probably represents the rudiment of the supra-cesophageal 

 ganglion. A ventral nerve cord is stated by Salensky to be 

 present, but his observations on this point are not quite satis- 

 factory. 



The metamorphosis is accompanied by the loss of swimming 

 power, and consists in the 

 enlargement of the post-oral 

 portion of the trunk, and in 

 the simultaneous reduction 

 of the prae-oral lobe, which 

 remains however perma- 

 nently as the cylindrical 

 proboscis. A groove which 

 terminates posteriorly at the 

 mouth is very early formed 

 on its ventral side. The 

 ciliated rings gradually dis- 

 appear during the metamor- FIG. 161. LARVA OF ECHIURUS. 

 phosis. (After Salensky.) 



.^ r ., r ,1 m - mouth; an. anus; sg. supra-oeso- 



Of the further external phageal ganglion (?). 



