OBSERVATIONS OX FKESH-WATEK i'OLYZOA. 135 



ridge runniDg parallel with, but less extensive than, the first one, 

 developes on the anal side of the mouth. Its extremities soon meet 

 and fuse together with the limbs of the first-formed semicircular ridge. 

 Tentacles are formed on the new ridge in the same way as described 

 above ; the range of their row extending on either side to tips of 

 lophophoral processes. Thus the inner row of tentacles is established 

 on the lophophore. 



The hollow process sent up by the stomach grows larger, and 

 finally its cavity opens into the u]>per clinmber or the tentacular 

 slieath, Avhich, when evaginated, forms the tubular body of the 

 polypide. 



The account given above may suffice to show how the general 

 shape of a polypide is formed in the contents of a stntoblast. In the 

 meantime rudiments of many other organs, of which the brnin, the 

 muscles, and the funiculus are the most important, have begun their 

 development. 



The cerebral "anolion arises as a pit-like invaoination of the inner 



CO i. ~ 



layer of the oesophageal wall, which is continuous with tlie outer 

 layer of the body- wall. The process begins to take place at a stage 

 when the stomach sends up the process that afterwards becoines the 

 intestine, on the anal side of the oesophagus, just inside the mouth. 

 The invnjjfination is soon constricted off, turning" it into a closed sac, 

 wliich as it is being formed, carries with it the outer layer of the 

 oesophageal wall, so that the latter invests it externally, at the same 

 time connecting it with the oesophagus. The cavity of the sac per- 

 sists as a sort of ventricle. The lower portion of the wall of the ^ac 

 early begins to thicken, which process does not of course concern the 

 investing layer, and finally developes itself into that portion which 

 constitutes the main ganglionic mass (vide p. 115). The remaining 

 portion of the sac- wall, except at two points, becomes thinner and 



