152 THE OCEAN WORLD. 
at the extremity of a trumpet-like prolongation, whitish and con- 
tractile, a sort of central mouth, surrounded by tentacular cirri, the 
external row being much longer than the internal ones. This was 
formerly thought to be the stomach of the Velella. In the present 
day, this appendage is known to be the central polyp around which 
are grouped other whitish and much smaller appendages, the base 
being surrounded by little yellow bunches. These are supposed to 
be the reproductive organs. Between the crest and the shield 
Fig. 50.—Velella spirans (Lamarck). 
numerous free tentacles present themselves, vermiform in appearance, 
cylindrical, and of a sky-blue colour, which are kept in continual 
motion. 
The Velella is therefore not an isolated individual, but a group or 
colony, in which the individuals intended to be reproductive are the 
most numerous, and occupy the inferior parts. 
The central polyp, by its size and structure, is distinguishable at 
the first glance from all the other appendages of the lower surface of 
the body. It is a cylindrical tube, very contractile and spear-shaped, 
swollen into a round ball, or considerably elongated. Its mouth is 
round and much dilated ; it opens in the cylindrical or trumpet-like 
part, which is contained in a sac, clothed in the whitish integuments 
