450 G. H. PARKER AND E. G. TITUS 



subjected by the changes in pressure within the animal. The 

 neuromuscular mechanism of the column wall in Metridium 

 consists, then, not only of a nervous arrangement by which dis- 

 tant muscles can be called into operation but also of a system 

 of muscle fibers acting under direct stimulation. Whether the 

 circular muscle is at all under the influence of nerves cannot be 

 definitely stated, but certain peristaltic movements that it shows 

 in recently fed individuals (Mtillegger, '13) suggest that it is 

 under more or less nervous control. 



The pedal disc of Metridium shows many of the same neuro- 

 muscular peculiarities that the column wall does, but we have 

 not had the opportunity to work out its organization to the same 

 extent as we have that of the wall. The ectoderm of the disc 

 is thicker than that in the column and is more richly provided 

 with gland cells. Its supporting lamella contains a system of 

 neurofibrils exactly like those in the column wall. These fibrils 

 take in general a circular course concentric with the center of 

 the disc and can be traced in among the circular-muscle fibers, 

 the basilar muscles, and even into the bases of the mesenteries 

 themselves. The cellular relations of these fibrils are as obscure 

 as are those of the fibrils in the column wall. Whether these 

 fibrils control the musculature of the disc or of the mesenteries 

 or both was not determined. Their presence as mesogloeal ele- 

 ments is the chief fact that we have to contribute to the anatomy 

 of this part of the actinian. 



In the oral disc, the lips, and the oesophagus we were unable 

 to obtain good impregnations, and we got evidence of only a 

 few circular neurofibrils in the supporting lamella of the oral disc. 

 The nervous layer of the tentacular ectoderm can easily be traced 

 in ordinary preparations into that of the ectoderm of the oral 

 disc, from which it can be followed over the lips and into the 

 ectoderm of the oesophagus. This is the region that has been 

 regarded by the Hertwigs ('79-80, p. 50) as centralized in func- 

 tion and the layer which can be traced into the oesophagus is 

 the beginning of the connection with the entodermic nervous 

 system. That the ectoderm of the lips and the oesophagus 

 in Metridium exhibits such a layer cannot for a moment be 



