HYDEOIDS 179 



at least a subumbrellar band. In general the polyp of 

 Corymorpha exhibits the arrangement of muscles found 

 in the typical hydrozoan, a longitudinal ectodermic sys- 

 tem and a circular entodermic one. The longitudinal en- 

 todermic system claimed by Lipin (1909) for Poly podium 

 is associated in that hydrozoan with a curious inversion 

 of the germ layers that may lay this interpretation open 

 to question. 



The functions of the several muscles already enumer- 

 ated for Corymorpha may be studied best by beginning 

 with those of the stalk. When a fully expanded, quiescent 

 Corymorpha is vigorously stimulated mechanically, its 

 stalk shortens to about one-half its former length (Tor- 

 rey, 1904 b) and at the same time thickens. Thus an ex- 

 panded polyp whose stalk measured 4.7 centimeters in 

 length on contraction had a length of only 2.5 centi- 

 meters. In ten such instances the average contracted 

 length was 57 per cent, of the average expanded length. 



If the hydranth is cut off from Corymorpha, the con- 

 tractility of the stalk remains essentially unaltered. 

 Thus a stalk without hydranth that had an expanded 

 length of 4.2 centimeters had on full contraction a length 

 of 2.3 centimeters and in ten instances of this kind the 

 average contracted length was 55 per cent, of the average 

 expanded length. Thus the contraction and expansion 

 of the stalk are quite independent of the hydranth. 



If the activity of the whole stalk is studied, the con- 

 traction is seen to occur in only that part which is ordi- 

 narily above the mud, the half centimeter or so of buried 

 stalk being incapable of longitudinal contraction. This 

 portion is devoid of longitudinal muscle fibers and hence 

 should be expected to be non-contractile, whereas such 

 fibers are invariably present in that part of the stalk dis- 



