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. Ve'ella 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 



