542 



SIPHONOPHORA ; VELELLA. 



contractile, so that they are able to draw the prey which they 

 have attacked towards their point of origin. This animal is one 

 of those most commonly observed by voyagers, sailing in fleets 

 upon the calm surface of the ocean, and disappearing with great 

 rapidity when alarmed by the roughness of the waves. 



1195. The Chondrograda, or Cirrigrade Acalephce, are dis- 

 tinguished by the possession of an internal skeleton, in the form 

 of a cartilaginous plate imbedded in the soft disc which forms the 

 upper part of their bodies. This plate is cellular in its structure, 

 and its cells are filled with air, an arrangement by which the 

 animal is assisted in floating at the surface of the water. 

 Amongst the most remarkable forms of this group is the Velella, 

 in which the disc is furnished with a delicate crest of a sub- 

 cartilaginous texture, which serves as a regular transparent sail 

 when the animal floats at the surface of the sea. One species of 

 Velella is found on the Irish coasts. 



