356 
REVIEWS. 
c. — u Pelagia cyanella lias already been accurately described by 
Escbscboltz, but a figure with details of its structure was still 
wanted,” and Professor Agassiz has here “ attempted to supply the 
deficiency.” The single specimen which he observed at theTortugas 
Islands, in the Gulf of Mexico, happily afforded him an opportunity 
of tracing, for five days, its early development, and his account of 
this, so far as it goes, is valuable, as tending to corroborate the fuller 
observations of Krohn on the European P. noctiluca. In neither of 
those species is a hydra-tuba stage noticeable. The young is directly 
developed, and is, at all periods of its existence, free swimming. 
d. e .—The two remaining species of North American Piscophorce 
here noted belong to the Phizostomidce. The very remarkable poly- 
stomism of these Medusce Professor A gassiz, as we have before hinted, 
explains by referring to those changes which take place, with increase 
of age, in the genus Aurelia :— 
“ As Aurelia grows older, the arms beeome thicker along their centre, and the 
thin margins are folded against one another, their edge alone remaining pliable 
upon the sides of the stiffer axis , but as these edges are themselves wider, longer, 
and more spreading than the axis, they fold, bend, and twist in every direction, from 
both sides, until, at last, these winding folds become also harder and stiffer, and 
can neither be fully opened nor stretched, so that, though the margin of the 
arms is free and open, from tip to base, and can be laid out like a flat leaf, with 
comparatively little effort, each arm of an adult Aurelia forms, in reality, a system 
of hat channels, gaping along the margin, and uniting into fewer and fewer ramifi¬ 
cations toward the middle line of the arm, along which runs the larger channel 
which terminates in the mouth. The central aperture, or the mouth itself, under¬ 
goes identical changes. Its walls become thicker and stiffer, and less movable, 
and are finally thrown into such folds as fit one against the other so closely, that, 
in the end, the oral aperture is transformed into a system of capillary surfaces, 
between the folds of the actinostome, leading into the main cavity.” 
“ Now such is exactly the structure of a Rhizostome, with this exception only, 
that the margins of these capillary surfaces interlocked with one another, are 
soldered up, and present, only at intervals and in particular places along the edge, 
which vary in different genera, apertures which through life remain open and keep 
up a communication between the surrounding medium and the main cavity, and 
through which the food necessary for their sustenance is absorbed. I know, from 
direct observation of the young of Polyclonia frondosa, one of the earliest Rhizo- 
tomidas known to naturalists, that in this species at least, the young has a simple 
funnel-shaped mouth, as widely open, as freely gaping, and as directly communis 
eating with the central cavity of the body, as in the young Aurelia and the young 
Pelagia. I know, further, that in more advanced young the angles of the mouth 
begin to project, in the shape of arms with open and free margins, as in Aurelia, 
Cyanea, and Pelagia. And though I have not actually seen the margins of the 
mouth of any specimen of this species grow together, in such a manner as to close 
up the mouth, yet the fact, that in a more advanced stage of growth, specimens 
found together in the same shoal, and in no way differing from one another in other 
respects, have the margins of the arms and of the edges of the mouth so united, at 
intervals, that they cannot be spread out or easily opened without tearing, as well 
as the additional fact, that in still older specimens, not, however, exceeding one or 
two inches in diameter, the extent of the union of the edge of the mouth is so 
great, as to leave only comparatively few passages for a free communication of the 
surrounding medium, with the main cavity of the body, shows most unques¬ 
tionably that the seeming absence of the mouth in Rhizostomete is only the result of 
a gradual closing up of the margins of the actinostome, which takes place, sooner or 
later, and to a greater or less extent, in different genera.” 
