GONIONEMUS. 25 



GONIONEMUS. 



As has been seen, the medusse buds are usually produced from 

 the walls of a specialized hydranth (Leptomedusae) or from the 

 manubrium wall of an ordinary hydranth (Anthomedusse). A 

 series of these buds in various stages would show the formation 

 of the umbrella-shaped individual with the growth of the marginal 

 tentacles around the edge. The life-history of this form is not 

 known, but from its structure we are led to believe that it 

 belongs to the suborder Leptomedusae.^ It is found in con- 

 siderable numbers throughout the summer in the border of eel- 

 grass in the Eel Pond at Woods Holl, where it may be obtained 

 with a dip-net. It is more satisfactory to study than the medusa 

 of Obelia, as it is much larger and its movements and organs 

 are more easily observed. In plan of structure the two are 

 quite similar. 



Put a living specimen in a flat-sicled jar containing sea-water, 

 or in a finger-bowl, with a black tile beneath, and notice: 



1. Its method of locomotion. To the contraction of what 

 part of the bell is movement due ? How large is the jet of water 

 that is delivered from the bell ? Why is the jet made narrow ? 

 Does the jet necessarily leave at the center or may it be thrown 

 from one side ? vShould it be thrown from one side, what would 

 be the result ? 



2. Its position in the water when quiet. Why is this position 

 more desirable than the opposite? With a needle-point prove 

 that various parts of the body are sensitive. 



With either fresh or preserved material notice : 



1. Its flattened dome-shape. The convex face is called the 

 ex-umhrella (aboral), while the concave portion is termed the 

 suh-umhrella (oral). 



2. The velum is the perforated diaphragm that partly closes 

 in the sub-umbrella. All medusse possessing this structure 

 are classed as Craspedota. Do you understand its use ? 



^ There is some reason for thinking that the polyp stage of this form 

 develops directly into the medusa. (See Perkins, Proc. Acad. Nat. Sci., 

 Philadelphia, 1902.) 



