HYDROIDS FROM WOOD's HOLL, MASS. 357 



ou the tentacles, than is usually the case in older medusae. 

 Ova were present on the manubrium. 



Older medusa (cf. fig. 11) measure from 1 to 2 mm. in 

 diameter; the umbrella is obovate, deeper than broad, the 

 walls rather thin. Just opposite the four radial canals on the 

 outside of the bell, and extending up about one fifth its 

 meridian, are swellings (fusiform sacs of Allman, p. 224), 

 filled with large nettling organs. The manubrium, extending 

 through over half the length of the bell, is cylindrical, becom- 

 ing conical when its walls are distended with sexual products. 

 The velum is well developed, with opening rather small. The 

 two tentacles on opposite perradii are quite long and slender 

 when fully expanded, and are provided with long slender 

 filaments, bearing thick-walled oval capsules, each of which 

 contains from three to five oval glistening bodies, and is beset 

 by stiff hair-like processes. At least the proximal portion of 

 each tentacle is hollow, as is evident from the circulation of 

 food particles, while farther out there appear to be separate 

 vacuoles, each containing minute granules exhibiting active 

 Brownian movement. The bases of the two tentacles are 

 enlarged, and bear irregular pigment masses. At the two 

 remaining perradii are slight prominences (below the swellings 

 before mentioned) filled with small nettling organs and some 

 pigment, representing, no doubt, two rudimentary tentacles. 



The very unique feature of the genus Gemmaria, as Allman^ 

 has pointed out, is the stalked organs on the tentacles. 

 These organs one is tempted to compare with the nettling 

 batteries of some of the Siphonophora, not only on account of 

 their containing a number of nematocysts ^ in one receptacle, 



» Ibid., p. 225. 



^ Although each nematocyst showed a ceutral body looking like folded barbs, 

 1 was at first iucliued to doubt their nettling function ; for while all other 

 nematocysts of the medusa responded to mechanical or chemical stimuli, these 

 were most obdurate. But finally I succeeded in causing threads to be dis- 

 charged, and now the evaginated thread showed that the appearance in the 

 intact capsule was due to a number of small folded barbs occurring just below 

 a vesicular enlargement of the thread (cf. fig. 13). Allman (ibid., pi. viii) has 

 also figured such a nematocyst from his Gemmaria polyp. 



VOL. 42, PART 3. NEW SERIES. A A 



