HYDROIDEA. 27 



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. Without 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 branchiae, — 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. Without 

 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. Without 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. 



