HYDROMEDUSAE 287 



about five tentacles on each bulb, the stomach develops perradial lobes, and at the same 

 time the peduncle begins to appear. The peduncle gradually increases in size, its base 

 occupies the v^^hole of the top of the umbrella cavity, and it hangs down, in the adult, 

 like a broad inverted cone, about two-thirds as long as the umbrella cavity. The lobes 

 of the stomach extend along the whole length of the peduncle, but never beyond it. 



The mouth is comparatively small in all stages. When contracted it has four short 

 perradial lips, but when expanded it is usually quadrangular, but sometimes circular. 



The oral tentacles are always four in number, perradially situated, and their base or 

 root is at the bottom of the stomach, a little above the mouth (Plate XV, fig. 14). In the 

 earliest stage the oral tentacles are unbranched, and at the end of each one there is a 

 small terminal cluster of nematocysts. As the medusa grows the simple oral tentacle 

 bifurcates and becomes once dichotomously divided. The dichotomous branching 

 continues (Plate XV, fig. 12), and each tentacle becomes like a little bush. After a time 

 lateral branches appear and these become subdivided into numerous smaller branches. 

 The growth is complicated to follow, but the diagram (Plate XV, fig. 13) shows how bush- 

 like the oral tentacles become and how numerous are the terminal branches, each having 

 a small cluster of nematocysts. In a fully developed adult the four oral tentacles have 

 between them about 400 terminal branches, a deadly entanglement for small crustaceans. 



The radial canals are fairly broad and conspicuous, running direct from the lobes of 

 the stomach to the compound basal bulbs. The ring canal is smaller than the radial 

 canals. 



The gonads begin to appear round the stomach and upon its lobes when the medusa 

 is about 2-4 mm. high. At first they form a mere swelling, but gradually increase in 

 size. In the large adults they form a folded ribbon-like band which extends along the 

 whole length of the peduncle and hangs down below the level of the mouth. Some 

 specimens have free ova inside the umbrella cavity, and the ova are segmenting. 



The compound basal bulbs show a considerable change in shape and appearance as 

 the tentacles increase in number. In the earliest free-swimming stage, o-5-o-75 mm. 

 high (Plate XV, fig. 8), the medusa has two or three tentacles on each of the four basal 

 bulbs, which are globular or spherical in shape. In the early intermediate stages, 2-4 mm. 

 high, the bulb broadens and becomes heart-shaped and has now five to ten tentacles in a 

 single row. As the tentacles increase in number (Plate XV, fig. 10) the sides of the bulb 

 increase in length and curl slightly over, and the whole bulb becomes U-shaped and 

 finally in the largest adults V-shaped. In some bulbs the sides of the V nearly meet, 

 but this may be due to a general contraction of the umbrella. 



When the medusa is about 4-6 mm. high and has about ten to fifteen tentacles on 

 each basal bulb, an inner series of tentacles begins to appear, starting at about the fourth 

 or fifth tentacle of the outer series, away from the apex of the U or V; in this way a 

 double alternating row of tentacles is formed (Plate XV, fig. 11). This inner series is 

 formed partly by a pushing inwards of the tentacles belonging to the outer row, partly 

 by addition of new tentacles in both of the two rows. In most other species of this 

 genus new tentacles are only developed at both sides of the basal bulb away from its 



