868 THE AMERICAN NATURALIST. [VoL. XXXIII. 
it did not stain so deeply. Many fine filamentous threads 
extended from the sense body to the walls of the tube. These 
threads seem to suspend it in the center. The fact that they 
are in such a rudimentary condition, and less highly specialized 
than in other Medusz, would tend to show that their function 
is of a simple character, possibly used more as balancing organs 
than anything else. After the chymiferous tubes are formed, 
the further changes which take place are chiefly confined to the 
reproductive elements and will be explained in speaking of their 
development. 
The description of the external morphology of the Medusa 
has been given so accurately by Agassiz that we can do no 
better than quote his exact words: “The Medusa of P. tiarella 
is one of the most remarkable of our naked-eyed Meduse. As 
in the Sarsiadz, the Medusa bud is formed among the tentacles. 
The chymiferous tubes never have the extraordinary thickness 
which is noted in Sarsia, and the cavity of the bell is hollowed 
out at an earlier period; the Medusa bud gradually becomes 
more elongated, and when mature is about one-sixteenth of an 
inch long.” The Medusa full of eggs measures 1.2 mm. long 
and .7 mm. wide. Free of eggs, .g mm. long and .§5 mm. wide. 
He continues by saying that “the walls of the spherosome are 
so thin that the Medusa will often assume a quadrangular or 
octagonal outline with deep indentations between the chymifer- 
ous tubes. Large white eggs fill the cavity of the bell; as they 
increase in size they give the Medusa an opaque milky appear- 
ance. The walls of the spherosome become thinner and thinner, 
and when the Medusa bud has attained its full development 
and is ready to be separated, the walls become so thin that the 
Medusa is almost always distorted, either on one side or the 
other, by bunches of spermaries or by the eggs which have 
reached such a great size that four or five of them completely 
fill the inner cavity.” The eggs are large white bodies, very 
opaque, .33 mm. in diameter. When they begin to segment, 
they look much whiter, and one can distinguish the segmenting 
egg by the unaided eye (2). 
The opening through which the reproductive elements are to 
be expelled is formed after the Medusa is mature. More obser- 
