HYDROZOA. 



105 



enclose 



single large polypite, nearly central in position ; 

 (2), numerous small gonoblastidia, which resemble' 

 polypites, and are termed * phyogemmaria ' ; and, 

 (3), the reproductive bodies to which these last 

 give rise (6). The tentacles are attached, quite 

 independently of the polypites, in a single series 

 along the line where the firm part and limb of 

 the disc unite. There are no hydrocysts, necto- 

 calyces, or hydrophyllia. The average length of 

 Velella may be estimated at two inches, its 

 height at one inch and a half. The entire or- 

 ganism is semi-transparent and tinged with an 

 ultramarine blue, which changes to a deeper shade 

 in the tentacles and limb of the disc. 



On closer examination the firm part is seen to 

 ' a hard, shell-like, pneumatocyst, con- 

 sisting of a horizontal division, included within 

 the disc, and continuous with the simple solid 

 vertical plate, which gives support to the sail 

 or crest. The upper surface of the pneumatocyst 

 is crossed at right angles to the direction of the 

 crest by a linear diagonal groove, indicated on its 

 under surface by a slightly elevated ridge, " while 

 a longitudinal depression, increasing in depth from 

 the margins to the centre, corresponds with the 

 attachment of the crest. The horizontal division 

 of the pneumatocyst consists of two thin lamina?, 

 passing into one another at their free edges, and 

 united by a number of concentric vertical septa, 

 between which are corresponding chambers filled 

 with air. All these chambers communicate toge- 

 ther by means of apertures in the septa. Of these 

 each septum presents two, placed at opposite 

 points of its circumference, and all nearly in the 

 middle line of the pneumatocyst. Kolliker made 



