92 The Siphonopkores. [February, 



much more strongly than corresponding structures of Agalma 

 and kindred forms. In this retracted position they are often car- 

 ried, as the animal darts forward in its course through the water. 

 To facilitate that motion by diminishing the resistance of the sur- 

 rounding medium, the method of attachment no less than the 

 form of the bells, contributes. 



In Agalma the nectocalyces, as we have seen, seem to arise in 

 two rows, with bell openings looking in opposite directions. 

 They are capable of a very limited change of position, and most 

 of the variety of motion which the colony has, is brought about 

 by combination in the action of nectocalyces situated in different 

 regions of the stem, or in a muscular twisting of the axis upon 

 which they are fastened, by which their openings are made to 

 face in different directions. The method is too simple if rapid 

 motion be desired, and ill adapted to that purpose in Diphyes. 

 In Stephanomia variety and rapidity of movement are brought 

 about by multiplication of nectocalyces. Even in this genus the 

 means are inferior to those which we find in Diphyes. 1 



The swimming-bells of Diphyes are placed one behind the 

 other, so that their longer axes lie in a straight line which falls in 

 the direction of motion. Both bell cavities open in the same 

 way, facing backward as they float in the water. When they act 

 simultaneously the fluid ejected from their cavities by the contrac- 

 tion of the bell walls, presses together on the surrounding me- 

 dium and reinforces each other. There is no action of one bell 

 in opposition to another, as may happen in Agalma. The vol- 

 ume of ejected water is comparatively much larger than in any 

 of the Physophoridse. 



The anterior bell (ab) of the two nectocalyces has a pyramidal 

 shape, and is pointed at the apex opposite the bell opening. If 

 this bell were attached by the same region as the nectocalyx of 

 Agalma, it would seem as if this apical prolongation should also 

 indicate the place of attachment of the stem. In Diphyes, how- 

 ever, this is not the case. The apex of the first bell is not homol- 

 ogous to the apex of the bell of other Medusae, nor does it cor- 

 respond to the point of attachment of the nectocalyx to the stem 



l The motion of the Diphyes is sometimes so rapid that the eye cannot follow the 

 animal. The water is driven out of the bell cavity by a single muscular contraction 



