342 PARKER— TYPES OF NEUROMUSCULAR 



of a sea-anemone as it is in normal acontia. I therefore conclude 

 that the acontial muscle is one that is normally stimulated directly 

 and that it is without nervous connections. In this sense it repre- 

 sents primitive muscle unassociated with nervous tissue as has 

 already been identified in sponges (Parker, 1910). 



A second type of muscle is that seen in the circular muscle of 

 the column of Mctridium. When a portion of the column of this 

 animal is fully anesthetized with magnesium sulphate, a mechanical 

 stimulus will not elicit from it the usual contraction of the animal as 

 a whole, but a ring of contraction will extend more or less com- 

 pletely around the column. When the column is not anesthetized 

 and the sea-anemone is vigorously fed, this muscle exhibits 

 peristaltic movements which apparently depend upon nervous ac- 

 tivity. The circular muscle of the column, therefore, seems to be a 

 muscle open to direct stimulation and also under the control of 

 nerves. In this respect it resembles the sphincter pupills of the 

 vertebrate eye which responds not only to nerve impulses but also 

 directly to light. 



A third type of muscular mechanism in sea-anemones is seen in 

 the longitudinal muscles of the mesenteries. These contract when 

 almost any part of the surface of a Metridium is stimulated, but 

 they fail to respond when the animal is deeply anesthetized. Hence 

 I conclude that they are primarily controlled by nerves. Under 

 ordinary stimulation their action is profound and lasting and is one 

 of the most important elements in the general contraction of the 

 animal as a whole. It is their activity that in large part has given 

 grounds to the idea that the actinian muscle is specialized almost 

 exclusively on the side of its tonicity. 



Finally a fourth type of muscle mechanism is that seen in the 

 transverse muscles of the mesenteries, particularly of the complete 

 mesenteries. When food juice is discharged on the tentacles or lips 

 of an expanded Metridium, the transverse mesenteric muscles con- 

 tract and thus open the oesophagus preparatory to what t:nder 

 normal circumstances would be the swallowing of the food. On 

 withdrawing the stimulus these muscles quickly relax and the 

 oesophagus closes. This reaction is so definite and precise in its 



