Biological Studies on Corymorpha. 403 



point of stimulation and away from the mouth. These move- 

 ments, together with the tendency of the proboscis to bend toward 

 the point of stimulation, carrying the distal tentacles with it, 

 undoubtedly supplement the movements of the proximal tentacles 

 in bringing food to the mouth, and raise the average of efficiency 

 of the prehensile mechanism. 



The contractions of the stem muscles, determining a limited 

 range of movement for the hydranth, may be of advantage to 

 Corymorpha. The rapid shortening of the stem following strong 

 stimulation, however, can have no value as a part of the 

 mechanism of prehension, nor does it have any apparent useful- 

 ness as a means of defense against predatory enemies. 



h. Geotropts?n; Functions of the Axial Endoderm. 



Up to this point we have been considering the effects produced 

 by the contractions of muscles, "^ in tentacles, proboscis and column, 

 under certain sorts of stimulation. 



We may now consider another type ot motor reaction induced 

 by another sort of stimulus which appears ultimately to affect 

 another tissue element. This is the tendency of the stem, in 

 assuming its most characteristic attitude, to turn directly away 

 from the center of the earth, by what seems to be a change in 

 the turgidity of the axial endoderm cells incited by the stimulus 

 of gravity. 



It will be unnecessary to enter into an extended discussion of 

 the phenomena of geotropism," which are familiar to all. A few 

 words will suffice for the purposes of this paper. 



^There are both longitudinal (ectodermal) and circular (endodermal) muscle 

 fibers in both proximal and distal tentacles, proboscis and column. As might be 

 expected from their activities, the circular fibers in the proximal tentacles form a 

 much weaker sheet than the longitudinal, except where each tentacle joins the body 

 of the hydranth. There the circular fibers are aggregated into a strong bandlike 

 sphincter, and there the tentacles are wont to break away from the hydranth under 

 unfavorable conditions. Such a habit of casting the tentacles seems to be character- 

 istic of certain anemones, notably Bolocera, and is accomplished by a similar 

 sphincter. 



^Davenport has distinguished between the responses of free and fixed organisms 

 to gravity, following Schwartz in applying to the former the term "geotaxis," and 

 applying to the latter the term " geotropism." With the facts which follow in mind, 

 it will be difficult, I believe, to see any advantage in this distinction, and it has 

 accordingly been disregarded. 



