454 WALTER GARSTANG. 



curve ventrally. The edges of the dorsal and ventral arms 

 are bordered by the prolongations of the post-oral and prse- 

 oral ciliated bands respectively. The length of the prse-oral 

 appendage and of the unpaired arms which arise from it in- 

 creases with the age of the larva, and increases relatively to the 

 general growth of the body. At the stage represented in fig. 

 1 the body of the larva has almost attained its full develop- 

 ment; but the prse-oral appendage, on the other hand, is not 

 fully grown. The increase in length of this appendage is 

 most marked in the case of the stalk and of the dorsal arm. 

 I am not in a position to say whether the prae-oral lobe ever 

 reaches the extraordinary dimensions it possesses in the case 

 of Sars's Bipinnaria, but such a development is quite within 

 the bounds of possibility. 



The larva drawn in fig. 1 was not quite my largest specimen. 

 In the preserved condition its dimensions were as follows : 



Length from apex of prostomium (x) to tips of posterior arms rS mm. 

 Length of anterior dorsal arm from apex of prostomium (x) 



to tip . . . . . . .0-9 „ 



Maximum length, therefore . • . . . 2*7 „ 



The thickness (dorso-ventral) of the prse-oral appendage was 

 0-2 mm. 



The internal structure of the larva may be gathered from 

 an examination of fig. 1. The alimentary canal is perfectly 

 normal, and needs no special description. On each side of it 

 lies one of the lateral enterocoeles (r. e.), which fuse together 

 immediately in front of the oesophagus to form the unpaired 

 prse-oral enterocoele {p. e.). The latter is attached to the 

 ventral wall of the prse-oral appendage for the greater part of 

 its course; but at the base of the unpaired ventral arm it 

 breaks away from the wall and sends a short process into the 

 dorsal arm. The left enterocoele communicates with the 

 exterior by means of a " water-tube " and pore, which occupy 

 their usual position on the left side of the stomach. The re- 

 stricted area occupied by the lateral enterocoeles posteriorly 

 points to the conclusion that these larvae have by no means yet 

 reached the final stage in their development. 



