TIIE HYHHOID MEDUSAE. 
341 
poljpite disguised in a dress which is suitable to the needs of a 
free existence. The general plan of structure will be best 
understood by a reference to the plate. It is of this kind. A 
delicate, more or less transparent disc or bell, serves as a float 
to which the various organs are attached ; it is eminently 
contractile, and by a regular systole and diastole propels 
itself through the water. In figure it is variable, but always 
bounded by lines of beauty ; sometimes it is almost globular, 
sometimes hemispherical, sometimes shaped like a watch-glass, 
and as translucent, sometimes of more fantastic form. Com- 
monly it is of the slightest, filmiest material, often colourless, 
and so crystalline as to be hardly visible ; as often tinted with the 
most delicate hues, which only the flower can match ; frail as 
the bubble, and brilliant as the bubble when touched by the 
sunlight. Within the cavity of the bell, and suspended from 
its summit (Plate LXXXVIIL, fig. 7, m), hangs the digestive 
sac , terminating below in a mouth , and at its upper extremity 
opening into a number of delicate canals (Plate LXXXVIIL, 
fig. 7, c , c), which traverse the walls of the swimming-bell longi- 
tudinally, and empty themselves into a circular vessel running 
round its margin. We have here the simple nutritive system, 
which corresponds essentially with that of the fixed and plant- 
like elements of the hydroid colony. The digested and 
diluted food is forced from the pendent stomach into the 
radiating canals, and conveyed by them and the circular 
vessel that unites them throughout the organism, the stream 
flowing back periodically to the central depot for fresh supplies. 
The number of the canals varies in different species ; com- 
monly it does not exceed four (Plate LXXXVIIL, figs. 3, 4, 6,), 
but six (fig. 3), eight, ten (fig. 7), and twelve are met with, and 
in some cases as many as a hundred (fig. 1). The vessels are 
generally simple, but occasionally they bifurcate or are slightly 
branched, and in one rare instance give off short, lateral diver- 
ticula ; never exhibiting, however, the remarkable complexity 
which occurs in the parallel group of the Discophores .* From 
the free margin of the bell hang a variable number of tentacles, 
some of which are a direct continuation of the radiating canals 
(Plate LXXXVIIL, fig. 3) ; these which are first developed 
and always present, may be regarded as the primaries, and the 
spaces between them are often occupied by large numbers of 
secondary tentacular appendages (figs. 1, 2,), forming a beautiful 
fringe of delicate interlacing threads. The tentacle is of course a 
prehensile organ and instrument of offence ; it is usually covered 
* Vide a paper by the author in the Popular Science Review for 
April 1871, in which the points of agreement and contrast between the two 
groups of the Hydroida and Discophoi'a are fully presented. 
