I'll) 



OF TI1K WOULD. 



S,-\ii t iHv-iiinlitn- nirJusa (plate 2O, figs. 9 and 10). Bell about 5 mm. in diameter and 

 slightly higher than a hemisphere with Baring, hell-shaped sides. Gelatinous substance 

 thick and rigid. There are 16 marginal tentacles, one at the foot oi" each terminal branch 

 ot' the radial-canals. 4 of these tentacles are at the ends of the 4 primary radial-canals. 4 

 younger tentacles are at the extremities of the secondary, 4 still smaller tentacles at the ends 

 of the tertiary, and 4 more at the ends of the quarternary canals. There are thus 16 marginal 

 tentacles, 4 in each quadrant. The tentacle-bulbs are large and hollow and their entoderm 

 is densely pigmented. The shafts of the tentacles are very contractile and capable of great 

 elongation. They are covered with ring-like clusters o( nematocysts, which become apparent 

 only during periods of contraction. There are neither marginal clubs nor cirri, but midway 

 between the tentacles, immediately under the surface of the exumbrella, are 16 slender, centrip- 

 etal canals which alternate with the tentacular radii in position. One or more clusters of 

 nematocyst-cells are found upon the exumbrella immediately over each ot these canals, and 

 according to Browne, 1904, these nematocysts travel singly up the canals from the bell-margin 

 and become lodged at intervals, forming accumulations. 



4 main radial-canals arise from the 4 radial corners of the stomach. Each of these 

 canals gives rise to 2 side branches and the longest and oldest of these side branches also gives 

 rise to a side branch. Thus there are 16 terminal branches each one of which ends in the 

 cavity of a tentacle-bulb. There is no ring-canal, but only a solid cord of entodermal cells 

 connecting the tentacle-bulbs one with another. The manubrium is flask-shaped and the 

 mouth is nearly at the level of the velar opening. There are 4 radially arranged, recurved 

 lips with crenated edges. The 4 gonads are developed upon the 4 interradial sides of the 



Fir.. loo. Diagrammatic oral views showing order of development of radial-canals in Prohosciiiacl\la ornala. 

 I, youngest stage; IV, mature condition. 



stomach. In the female the mature ova are large and prominent. The entoderm of the 

 stomach and gonads is ocher-yellow or greenish-yellow, while the entoderm of the tentacle- 

 bulbs is crowded with brownish pigment granules. In the very young medusa, however, the 

 entoderm of the manubrium and tentacle-bulbs is often pink. 



Toung nifJusa. The hydroid of our American species has not been observed. In the 

 youngest medusae seen by me, the bell is about 0.7 mm. in diameter (plate 20, figs. 1-4). 

 The gelatinous substance is relatively thinner than it is in the adult. There are 4 equally 

 developed tentacles, capable of much expansion (fig. 4), but these are usually seen contracted 

 in captive specimens. These tentacles are situated at the bases of the 4 primary radial tubes. 

 4 slender, blindly-ending, centripetal canals arise from the margin midway between the 4 

 tentacles, and there is a cluster of nematocyst-cells upon the exumbrella immediately over 

 each canal. The manubrium is a simple, short tube with 4 well-developed, quadratic lips. 

 The condition of the medusa in this stage is represented in text-figure 100, I, which repre- 

 sents an oral view of the animal. 



When the medusa is about 1.25 mm. in diameter ( plate 20, fig. 6), a side branch arises 

 from each of the 4 primitive radial-canals, and at the same time a second set of tentacles 

 (2222, text-figure 100, 11) make their appearance. These new tentacles are about 22 30' 

 away from the primary tentacles (iin). If one face the oral surface of the medusa, as is 

 shown in text-fig. 100, it will be seen that the secondary tentacles (2222) lie in positions 

 opposite to that of the hands of a watch relatively to the primary tentacles. 



The third set of radial-canals (3333, fig. 100, in) arise from the 4 primary canals beyond 

 the origins of canals 2222. This takes place when the medusa is about 2 mm. in diameter. 



