THE LARVA OF ASTERIAS VULGARIS. 117 



there flatten and form a discontinuous covering for these 

 organs. These mesenchyme-cells, in the later history of the 

 larva^ become differentiated into small fibres, which function 

 as muscle-fibres. This differentiation takes place very early 

 upon the walls of the entodermic portion of the oesophagus, 

 where they form a circular and a longitudinal layer. The 

 gulping movements brought about through the agency of these 

 cesophageal muscles are very violent, and take place at inter- 

 vals of fifteen to twenty seconds. A contraction starting 

 behind the mouth travels towards the stomach, accompanied 

 by a simultaneous longitudinal contraction of the oesophagus. 

 As the final act the end of the oesophagus at its union with the 

 stomach is violently pushed into the latter, and the contents 

 of the oesophagus, driven down in front of the circular constric- 

 tion, are suddenly belched into the stomach. 



Differentiation of the mesenchyme-cells into muscle-fibres 

 also takes place in the walls of the enterocoels, but no very 

 definite layers were made out. 



On the inner surface of the dorsal wall of the young Bipin- 

 naria mesenchymatous muscle-cells and fibres are seen ex- 

 tending from the dorso-lateral portion above the stomach 

 forwards along the median line (figs. 14 and 23, m. m.). 

 These fibres have probably to do with the very considerable 

 motion of which the preoral lobe is capable ; the motion is in a 

 dorso-ventral direction, and is accompanied by the formation 

 of two or more wrinkles of the dorsal surface at the narrow 

 part of the body of the larva. These fibres I judge to be the 

 same as those described by Semon (38) as bilaterally arranged 

 masses connected by a single commissure, and which he seems 

 a little inclined to consider as a part or the whole of the larval 

 nervous system, though he speaks of the possibility of their 

 being muscular in function. However, the fact that they 

 arise from mesenchyme-cells makes for the view that they are 

 muscular tissue. 



In the older larva (fig. 24) there is a small aggregation of 

 mesenchyme-cells, in the main posterior to and just to the 

 right of the pore canal. Its position is shown in fig. 18 {s. v.). 



VOL. XXXIV, PART II. — NEW SEE. I 



