84 LOWER INVERTEBRATES. 



four rudimentary ones. On the edge of the disk, at equidistant points, are a number 

 of globular bodies containing a cavity in which is a bristly ridge, and which is nearly 

 filled with a clear liquid, in which are a few small calcareous particles that strike 

 against the bristles when any disturbance in the water outside sets the liquid of the sac 

 in motion. These are known as otocysts, and are supposed to be auditory organs. 

 The medusse of Obelia longissima are very minute, measuring only one-sixtieth of an 

 inch across the disk, and one-fortieth across the outstretched tentacles. 



Fig. 76. — Cauipanularian liydroid; a, 6, hydranths; c, hydrorhiza; /, gonangium; g, medusa. 



A less conspicuous but very beautiful hydroid of special interest, and belonging to 

 the same family as Obelia, is represented by several species on the New England 

 coast. Tliey belong to the genus Gonothyrea, and, at a hasty glance, look like dimin- 

 utive or young specimens of Obelia. In height they do not exceed an inch and a half 

 or two inches ; the hydrothecte in the most common species, G. kyalina, are long, of 

 very thin texture, and the rim is cut into numerous shallow teeth of castellated form. 

 The gonothecae spring from the axils of the branches, and contain a blastostyle upon 

 which are formed a number of buds that de\'elop in regular sequence from above 

 downward ; when the uppermost one is fully grown, it pushes out of the top of the 



