ON THE DEVELOPMENT OF LINGULA ANATINA. 83 



(PL V., Fig. 78, and PL VII., Fig. 91, re.) which represents 

 the thinnest part of the arm-apparatus, and bulges out dorsally 

 when the animal contracts. It disappears when the arm-apparatus 

 is fully extended, helping to give a great length to the latter. 

 This recess is gradually reduced as the larva? grow, and in those 

 of the next stage this structure disappears altogether, leaving no 

 trace. 



Ventral to the mouth there is an area which forms the 

 ventral part of the arm-apparatus. In this portion the cells are 

 of the same nature as those of the oesophageal epithelium : the 

 cell-layer has become greatly reduced in thickness at the 7-9 p. c. 

 stage, as compared with the 3-6 p.c. stage (PL VIL, Figs. 91 

 93, *)• 



In the region of the arm-apparatus where the dorsal group 

 of the cirri are planted, there occur a pair of well marked ridges 

 projecting dorsally and recurving a little towards the median plane. 

 Between this and the oesophagus which (œs.) forms a median 

 longitudinal out-swelling of the body wall (PL VII., Fig. 95) 

 there occur a pair of tolerably deep grooves (PL VIL, Fig. 

 95, * *). 



The cavity in the arm-apparatus (arm-sinus) is directly 

 continuous with the body cavity at this stage, the blood corpuscles 

 freely flowing from one to the other. The communication takes 

 place by means of a pair of canals situated along the ventral side of 

 the oesophagus (PL VIL, Fig. 95, f f ). The arm-sinus in its turn 

 gives off the tentacular and cirrial canals. The sinus is partly 

 filled with trabecular mesenchy matous cells and with complicated 

 musculature. As the larva: 1 advance, these trabecular cells turn 

 into the epithelial lining of the arm-sinus. Between this lining 

 and the outer epithelium there comes to be found a strong layer 



