102 CLASS III. 
finished: the halves again divide in a direction perpendicular to 
the former line of division: but here the fission does not always 
begin in the stomach. How often the process may be repeated is 
not known. No trace of fission was observed in Mesonema, though 
it was often seen to occur in the larval forms, even when ova were 
already distinctly visible on the vessels. ] . 
[The Stphonophore have been shewn, by the ig eta of 
Huxuey, Levckart, KorLuikerR, GEGENBAUER and VoGtT, to be - 
compound animals, or colonies, connecting the hydroid polyps with 
the acalephs. They are named by KOELLIKER, in consequence, 
Swimming Polyps (polypi nechalet). ‘They consist in general of a 
stem, usually cylindrical and long (Diphyes), sometimes shortened 
and sacciform (Physalia), sometimes disciform (Velella), to which 
appendages are attached which differ remarkably in form and fune- 
tion. Some of these are suctorial tubes or stomachs, others motive 
organs, others feelers and prehensile organs, others again protective 
laminee (bracts) and sexual capsules. Great differences prevail with 
respect to the number, arrangement, and development of these parts, 
in the different families: those which are constant in all siphono- 
phors are the stomachs, the prehensile apparatus, and the sexual 
capsules. The stem is muscular, and hollow—the interior forming a 
canal in which the nutrient fluid moves with rapidity. The swim- 
ming apparatus is either passive or active—when passive it is a 
hydrostatic apparatus consisting of a bladder filled with air which is 
always placed at the upper extremity of the common stem: when 
active it consists of swimming-bells, which are also placed at the 
upper extremity of the stem, and are variously grouped, and in 
variable number in different genera: the swimming-bells may exist 
conjointly with the air-sac or without it. These bells are, in 
general, formed on the plan of a Medusa, consisting of an elastic 
bell-shaped mantle, very various in form, with an internal mus- 
cular layer which surrounds the swimming-sac. On the outer sur- 
face of the latter, there is a system of four radiating vessels, which, 
at the circumference of the aperture, fall into a circular vessel, and 
at the summit of the bell arise from a single vessel, which passes 
through the pedicle of the bell and falls into the cavity of the com- 
mon stem. All the other appendages of the stem have also a more 
or less perfect system of vessels, which communicate with the inter- 
nal cavity of the stem in a similar way. The only communications 
