84 



L WER IN VER TERRA TES. 



four rudimentary ones. On the edge of the disk, at equidistant points, ai-e a number 

 of gloI)ular bodies containing a cavity in which is a bristly ridge, and which is nearly 

 tilled 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 niedus£e of Obelia longissima are very minute, measuring only one-sixtieth of au 

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



-Cuuipauulaiianliydroid; it, i, hjJrantlis; c, liyj 



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. They 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 hydrotheca; in the most common species, G. /njalina, 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 numlter of buds that develo]i in regular sequence from above 

 downward ; when the uppermost one is fully grown, it jiushes out of the toji of the 



