THE DISCOPHOEES, OH LAEGE MEDUSJS. 
119 
type, but also frequently maintains its connection with the 
primitive stock in a less highly specialised form. So the Dis- 
cophore has its two equivalent elements ; a fixed polypite, 
whose functions are merely nutritive and vegetative, and a 
sexual zooid, developed from the former by gemmation, in 
which the Medusan structure reaches its highest grade, and 
which always leads a free oceanic life, while discharging its 
reproductive functions. In both groups there are exceptional 
genera in which the course of the life-history is modified by 
the suppression of the fixed element, and the Medusa is 
developed directly from the ovum, and not as a bud on a hydra- 
form stock. Such forms, however, exhibit in each case the 
closest affinity to those which are produced in a more normal 
manner, and should clearly take their place amongst them in 
the ranks of the same division. In. the two groups, then, the 
general plan of the life-history is identical, and there is a strik- 
ing similarity in many of the structural features. The differences 
will be noticed when I come to describe the organisation of the 
Discophore in detail ; but one salient point of contrast may be 
mentioned at once. Amongst the Hydroid Zoophytes the 
fixed vegetative element predominates ; the sexual members 
of the colony, though sometimes free and locomotive, are in a 
large number of cases as permanently attached to the parent 
organism as the flower to the plant, and where medusiform 
zooids are present, they are comparatively small, incon- 
spicuous, and of a lower structural grade. But amongst the 
Discophores, the locomotive medusan element is altogether in 
the ascendant and reaches its culminating point ; the polypites 
are small and insignificant, and of much the same pattern ; the 
more highly specialised structure has gained upon the merely 
vegetative ; and vagrancy, instead of plant-like fixity, is the 
characteristic of the tribe. 
Let me first define more precisely the limits of the'Order which 
forms the subject of the present paper. The Discophora em- 
brace the large jelly-fishes (Plate LXX. fig. 10), or Swimming - 
polypites — to borrow the expressive German name — whether 
developed directly from an egg or as buds from a hydra-like 
stock ; and also an aberrant group of fixed Medusae, the Lucer - 
nariidce , of which I shall have more to say hereafter. The 
jelly-fish is both the most characteristic and the most familiar 
form under which the Discophore presents itself to us ; and I 
shall at once attempt to sketch the leading features of its 
structure in plain as distinguished from technical language, 
merely premising that in a large proportion of cases it is not 
to be regarded as in itself a perfect animal , but only as one 
term of a life-series, which cannot be rightly interpreted alone. 
The feature which is most prominent and at once arrests 
K 2- 
