ON THE DEVELOPMENT OF LINQULA ANATINA. 37 



is more or less contractile ; for when the larvae are fatigued or 

 thrown into a fixing fluid it is sometimes separated from the shell- 

 margin by a strong contraction. 



Though the cirri have not increased in number as compared 

 with the oldest larvae reared from eggs, they are elongated and 

 in each a lumen becomes visible which I shall call the cirrial 

 canal (er. en.). The lumen is a diverticulum of the central arm- 

 sinus, and is formed by the flattening of mesenchyme cells to 

 line the cavity of the ectoblast. The cirrial muscle (m. er.) 

 is already formed, the fibers being produced from mesenchyme 

 cells (PI. IV., Fig. 71.). 



A pair of muscles, the ventral muscle, (m. vt.) (PI. IV., 

 Fig. 72, and PI. V., Fig. 74.) which have been seen in the 

 preceding stage, become more conspicuous. In parasagittal section 

 the course of the muscles can be clearly followed. They arise from 

 the ventral body wall a little posterior to the anterior edge of 

 the stomach and run forward, passing one on either side of the 

 oesophagus, and terminating on the dorso-lateral side of the latter. 



In two fixed specimens, a pair of black spots, one on each 

 side of the oesophagus, were seen by reflected light from the 

 ventral side ; by transmitted light each spot was resolved into 2-3 

 transparent and highly refractile crystall-like granules. Whether 

 they are the Anlagen of Oeelusores Anteriores or not I cannot 

 as yet decide owing to the lack of materials. 



At this stage there occurs a notable thickening along the 

 ventral margin of the arm-apparatus. This thickening forms the 

 ventral ridge of the arm-apparatus, and persists up to the 5-6 

 p. c. stage as is seen at the point marked * in Fig. 9L (PI. VII.). 

 Afterward the part is greatly thinned out. 



While the oesophagus shows no special changes, the stomach 



