6J4 



CILIA. 



Cilia appear also to exist in other tribes of 

 Medusae besides the Ciliograda, but they differ 

 in form and situation from those described, 

 and have not been investigated with equal 

 accuracy. 



In Rhizostoma there are certain membranous 

 appendages attached to the arms or tentacula, 

 and bearing on their free edge a fringe of short 

 filaments which are constantly in motion, and 

 continue so for some time after the arm or 

 portion of membrane supporting them is de- 

 tached from the body. These filaments are 

 described and figured by Eysenhardt,* who 

 regards them as organs of generation ; they are 

 probably of the nature of cilia. Similar fila- 

 mentary organs seem also to exist within the 

 body in some Medusae. (See ACALEPHJE, 

 p. 48.) 



5. Actinia. In a paper published on the 

 present subject in 1830,^ I mentioned that I 

 had found the ciliary motion in the Actinia or 

 Sea-anemony, but gave no description of it. I 

 have since re-examined various species of 

 Actiniae with this view, and shall now describe 

 the appearances ; but to make the description 

 intelligible, it may not be improper to remind 

 the reader of some points in the anatomy of 

 these animals which require to be kept in 

 view. 



The body of the Actinia, of which Jig. 297 



Fig. 297. 



Actinia* 



is a plan, consists entirely of a soft but tough 

 substance, exceedingly contractile and irritable. 

 It is usually cylindrical in shape, one end, 

 (a, ,) named the base or foot, serving to fix 

 the animal by adhering to rocks or other ob- 

 jects ; the other extremity is named the disc, 

 one-half of which is seen at 6, b, the other 

 half being removed by a section ; it is sur- 

 rounded at its circumference by the arms or 

 tentacula (c, c,) in concentric rows, and in its 

 centre is the mouth (d\ or opening of the 

 stomach, which serves both for the entrance of 

 food and discharge of undigested remains. 



Nova Acta Acad. Caes. Leop. vol. x. p. 404. 

 t Edin. Med. and Surg. Journal, vol. xxxiv. 



The stomach (e) is plaited longitudinally on 

 its inside; vertical membranous partitions 

 (.> S> g'j g>) pass from its outer surface to the 

 inside of the parietes of the body, and to the 

 base, dividing the intermediate space into 

 numerous compartments or cells, which com- 

 municate with each other by openings, as at 

 g', g', and also open into the tentacula, as at //. 

 The latter are conical muscular tubes, commu- 

 nicating at their base with the cells, and open- 

 ing at their point by a small orifice, surrounded 

 by a sphincter muscle. The cells seem also 

 to communicate with the cavity of the stomach, 

 and, according to Rapp,* they open in some 

 species by small orifices on the surface of the 

 body. The cells and tentacula contain sea- 

 water, with which the animal can distend the 

 whole body or any particular part of it. The 

 protrusion of the tentacula, as is well known, 

 is effected by their distension with water. The 

 stomach also is often partially everted and pro- 



_ & 



truded from the mouth by an accumulation of 

 water behind it. It has not, so far as I know, 

 been clearly shewn by which of the communi- 

 cating orifices the water enters. Though I 

 took considerable pains, I have not been able 

 satisfactorily to ascertain this point; I may 

 remark, however, that I have repeatedly no- 

 ticed water entering at the mouth. 



The ovaries and oviducts (/c, /c,) are lodged 

 in the cells, and are consequently bathed in 

 water ; of these it is unnecessary here to say 

 more than that one part of them consists of a 

 waving membranous fold like a mesentery, at- 

 tached by one edge to the sides of the cell, and 

 at its free border supporting the oviduct, 

 which resembles a white opaque chord, termi- 

 nating, after numerous serpentine windings, in 

 the stomach. 



In regard to the ciliary motion in the Actiniae, 

 I am led from my observations to conclude that 

 it exists to a greater extent in some species than 

 in others. In all cases I have found it on the sur- 

 face of the oviducts and their supporting mem- 

 branes, which is covered with cilia of very minute 

 size ; also on the internal surface of the sto- 

 mach, which has similar cilia, and there the 

 currents follow the direction of the folds of the 

 membrane. In one small but full-grown species 

 I found currents commencing near the centre 

 of the disc, and proceeding outwards in a 

 radiating manner to its circumference, whence 

 they continued along the arms as far as the 

 points. On examining this species, which 

 was semitransparent, by transmitted light, I 

 distinctly perceived moving particles in the 

 water contained within the tentacula and be- 

 hind the protruded stomach.f The motion of 

 these particles obviously indicated a current in 

 the water along the surfaces containing it, 

 which current, like that on the oviducts, it 

 may be inferred was produced by cilia, for it 

 went on while there was no perceptible con- 

 traction taking place in any part of the ani- 

 mal. The particles indicating the currents 



* Ueber die Polypen und die Actinien. Weimar, 

 1829, p. 47. 



t Some of these particles were no doubt the ova. 



