HYDROIDEA. Q7 
wasted polyps, the fluid of the trunk showing the only evidence of 
vitality by its continued vibration. And in the course of a few days 
other polyps have appeared in the vacated cells, with the same per- 
fection of form and the same activity and life as their predecessors. 
The polyp heads, as Dalyell states respecting a Tubularia, sometimes 
seem to drop off like a deciduous flower, and again, after ten days or 
more, are reproduced. Harvey observes, that after he had kept his 
specimens two days, they began to look unhealthy; and on the third 
“the heads were all thrown off, and lay on the bottom of the vessel.” 
After another three days, changing the water in the mean time, the 
polyps were entirely renewed, with no essential difference, except 
absence of colour. ‘The cold of winter is said sometimes to strip a 
corallum of its polyps, which remains thus apparently dead till 
spring, when it is warmed anew to life, and the polyp-flowers once 
more appear.* 
In conclusion, the Hydroidea are animals with no external organs 
but tentacles and a mouth, and no internal, but a simple stomach 
cavity and its prolongation below in the form of a tube or tubular 
axis. Waothout any special glandular system, and but a single opening 
to the alimentary cavity,—the food is digested by the gastric fluid of 
the stomach, and the refuse matter ejected by the mouth. Without 
a special absorbent or a circulating system or branchie,—the digested 
material of the stomach passes downward into the tubular axis, where 
it has a vibratory or cyclosis movement; and here it is farther elabo- 
rated by the action of air from the admitted water, and becomes 
absorbed and assimilated by the surface of the cavity, or of the tubular 
organs, cavities, or pores, connected with it—these chyloid fluids 
acting in place of a proper circulating fluid; aeration of the same 
also takes place through the tentacles and the exterior surface of 
the animal, which receive air from the waters about them. W/7thout 
ovarian glands, almost any part of the polyp possesses the reproductive 
function, excepting the tentacles; and buds or ovules are formed, 
and pass out directly from the sides of the animal. Wethout a distinct 
nervous system, in addition to the above negative characters, every 
part seems equally a centre of organic forces (unless we except the 
tentacles), and consequently sections made almost indefinitely still 
live and complete the entire polyp again. 
* J, G. Dalyell, Edinb. New Phil. Journ. xvii. 411 ; Harvey, Proceed. Zool. Soc. No. 
41, p- 55; Lister, Phil. Trans. 1834, 374, 376, 
