194 G. H. PARKER 



vorable features in the environment, next, the maintenance of 

 a normal metabohsm, and, last, reproduction. With this final 

 category we shall have nothing in particular to do; under the 

 second we shall take up the matter of feeding, and under the 

 first that of general retraction and expansion. 



The elements that are combined in most of these processes 

 have been discussed elsewhere. Actinians such as Metridium 

 possess at least four systems of effectors; slime glands, cilia, 

 nematocysts, and muscles, of which only the last gives evi- 

 dence of being under nervous influence, and even among these 

 certain muscles are very probably independent effectors. By 

 means of combinations of these elements, the various acts in 

 the appropriation of food, and in retraction and expansion are 

 accomphshed. These general activities will be considered on 

 the following pages in the order named. 



2. APPROPRIATION OF FOOD 



The appropriation of food is an activity with which the oral 

 disc of actinians is principally concerned. The movements of 

 the tentacles, mouth, and other such parts by which food is 

 ingested were ascribed by Nagel ('92, '94) to muscular action 

 alone, but Loeb ('95) pointed out that cilia also play an impor- 

 tant role. The parts that are immediately concerned in the 

 appropriation of food are the five following : the tentacular gland 

 cells, whose secretions render the tentacles adhesive whereby 

 pieces of food become attached to them; the musculature of the 

 tentacles, by which these organs are pointed toward the mouth ; 

 the tentacular cilia, which sweep toward the ends of the tenta- 

 cles and thus deliver the food to the mouth when the tentacles 

 are pointed in that direction; the transverse muscles of the com- 

 plete mesenteries, by which the esophagus is opened; and the 

 cilia of the lips and esophagus, which in the presence of food 

 reverse their usual outward stroke and thus transport such 

 materials to the gastro-vascular cavity. Beside these five sets 

 of parts some actinians include in the means by which they 

 appropriate their food a sixth system, namely, the musculature 

 of the oral disc. In Stoichactis, for instance, as described by 



