i64 AN INTRODUCTION TO ZOOLOGY. 



floating body, and is called tlie hydrosoma. It has a 

 float at the top, which contains a contractile air- 

 chamber, the use of which is to keep it upright, and to 

 raise it to the surface of the water, or depress it as 

 required : these are called the pneumatophore and 

 pneumatocy st. The nutritive zooids have tentacles full 

 of thread-cells : and besides the reproductive zooids, 

 there are sometimes protective and degenerated zooids 

 of several kinds. The Physalia, or Portuguese Man- 

 of-War, comes sometimes to our shore from warmer 

 seas. It receives its name from its appearance, as it 

 sails on the surface of the sea with its pnenmatophore 

 rising above the water. It stings virulently if 

 touched (Fig. 37). 



The AcalepJia, or Scyplwrnediisce., have no colonial 

 stage, but are usually developed from a larval form, 

 though they sometimes develop directly from the 

 ciliated embryo. The larval form is very peculiar ; it 

 first exists as a larva with tentacles called a ScypMs- 

 toma. From this a succession of buds with tentacles 

 are developed one within another, row after row, like a 

 fir-cone, and from its appearance the larva is called a 

 Strobila (fir-cone) : these buds, after further develop- 

 ment, split transversely away from one another and be- 

 come separate medusae. The Lucernaria is a small fixed 

 form of the same order, which may be found occasion- 

 ally attached to seaweeds, etc. : it is very like a small 

 eight-armed medusa inverted and fixed by the top of 

 the umbrella. It is therefore in the same position as 

 the polyps of the Tlydromedtisce, but does not in the 



