HYDR0Z0A. 



59 



N 







thickening. 





In a Velella, less than '1 of an inch long, ob- 

 served by Mr. Huxley, u the horizontal disc of the 

 adult was represented by a bell-shaped, mem- 

 branous expansion, continued above into a broad 

 crest, half as high as the whole depth of the 

 animal. It was symmetrically disposed, and its 

 superior edge, far from being pointed, was rather 

 concave, and in the centre presented a curious 



The central polypite was already 

 open at its distal extremity, and around its base 

 were a few short, csecal processes, the rudiments 

 of the gonoblastidia or of the tentacles. The 

 margin of the disc was occupied by a single 

 series of large, oval vesicles. The somatic cavity 

 was divided by a series of vertical septa, which 

 passed continuously over the pneumatocyst into 

 the crest, near whose free edge they terminated 

 abruptly, and between them other very short 

 septa were interposed. The somatic cavity and 

 its continuation into the crest were thus broken 

 up into a series of parallel [ciliated] canals, united 

 at their ends by two marginal canals at right 

 angles with one another, one in the disc, the other 

 in the crest. The pneumatocyst shone through 

 the disc, and did not extend into the crest at all." 

 It appeared "as an almost hemispherical body, 

 convex above and flat below. On two of its sides, 

 in a plane perpendicular to that of the crest, there 

 was a double crescentic mark, caused by a depres- 

 sion. The air did not completely distend the 

 pneumatocyst, but appeared to be divided into 

 seven or eight lobes below, so that, at first sight, 

 the organ itself appeared to be lobed, but this 

 was not really the case. It was, in fact, in the 

 smallest specimens a simple vesicle, about '05 of 



