HYDROZO^. 



Fig. 45. 



As it expands, it gives origin to the cells and branches characteristic 

 of the species ; and, from being at first quite soft and flexible, it gra- 

 dually acquires hardness 

 and solidity by the depo- 

 sition of corneous matter 

 in its substance. 



(221.) The cells thus 

 formed are inhabited by 

 polyps analogous to those 

 that provide nourish- 

 ment for the cortical fa- 

 milies, though differing 

 in the number and ap- 

 pearance of the tenta- 

 cula, which are here 

 studded with minute tu- 

 bercles, but never pro- 

 vided with cilia. Few 

 objects are more admi- 

 rable than these polyps, 

 when watched with a 

 good microscope. Pro- 

 truding themselves be- 

 yond the mouths of 

 their cells, they inflect 

 their bodies in all direc- 

 tions in quest of prey, 

 waiting till some passing object impinges upon their tentacula, when it 

 is at once seized and conveyed into the stomach with a rapidity and 

 dexterity almost beyond belief. 



(222.) The tentacula in the Sertularian Hydrozoa are all arranged in 

 a single row, and form a sort of funnel-like appendage to the oral orifice 

 of the body. They are susceptible of considerable elongation and con- 

 traction, like those of the Hydra, but in a less degree. Their number 

 is constant throughout the different periods of growth in each species, 

 but varies in different genera. Internally they are not hollow as is the 

 ease in the Anthozoa, but under the microscope they have the appear- 

 ance of being divided into compartments by delicate transparent dia- 

 phragms, giving them an appearance like that of some Confervse ; they 

 are throughout of equal thickness, and no movement of fluids is percep- 

 tible in their interior. 



(223.) In the centre of the tentacular circle may be observed a 

 fleshy protuberance of variable shape, which might be compared to a 

 proboscidiform elongation of the mouth ; sometimes this appendage is 

 elongated into the form of a tube, sometimes it shrinks into a globular 



Diagram representing section of a Sertularian zoophyte, 

 a, inner or nutritive layer; 6, outer or tegument?, ry layer; 

 c, oral tentacles of the polyp ; d, e, gemmules ; f, polypiform 

 external capsule ; g, polypiferous cell; h, reproductive cell. 



