GONIONEMUS VERTENS (A hydroid medusa). 



Phylum III, CCELENTERATA; class i, HYDROZOA; order I, 



LEPTOLIN.E. 



HABITAT. This small jelly-fish may be found, along the New 

 England coast, swimming about in tide pools or stretches of water in 

 which eel-grass is growing. Its life history is not well understood, but 

 it is probably a hydroid medusa budding off from plant-like masses of 

 fixed hydroid polyps. Gonionemus is not, however, a medusa-bud 

 from Pennaria. In turn it gives rise to egg- or sperm-cells which, 

 after fusion, develop into a sort of larva called a planula. After 

 swimming about for a time, the planula settles down, becomes 

 attached, and by growth and subsequent branching forms a new 

 colony of polyps. 



Technical Note. Put a specimen of Gonionemus in a small glass dish full 

 of water a deep watch glass will do and study with a hand lens or a 

 dissecting microscope. 



GENERAL APPEARANCE. Note the transparent jelly-like substance 

 of which the animal is composed. Not more than one per cent is solid 

 matter, the rest being highly organized water. The body is flattened 

 dome- or umbrella-shaped. The convex outer surface is generally 

 referred to as the ex-umbrella, while the concave portion is termed the 

 sub-umbrella. 



STRUCTURAL DETAILS: 



a) Depending from the center of the sub-umbrella, like the clapper to a 



bell, is the manubrium. 



b) At the free end of this organ is the mouth, opening into a cavity 



which occupies the whole interior of the manubrium and from a 

 dilation at its base sends off four tubes, the radial canals, to the 

 margin of the umbrella. 



c) Parallel with and close to the margin of the umbrella runs the 



circular canal. Into this the four radial canals open, the whole 



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