CILIA. 



619 



which the preceding account is founded, were 

 made more recently. There is no mention of 

 the existence of the phenomenon in the Aphro- 

 dita to be found in systematic works on com- 

 parative anatomy, nor in any of the special 

 memoirs on that animal which I have had an 

 opportunity of consulting. 



The ciliary motion exists in several other 

 animals belonging to the class Annelida. It 

 is remarkably distinct, and easily observed, on 

 the branchiae or gills of the Serpula. These 

 organs consist of two bunches of pinnated or 

 feather-like processes, which the animal pushes 

 forth from the calcareous tube in which it lives, 

 and spreads out in a radiating form. The 

 edges of the branchiae, both of the stems and 

 of the leaflets, are fringed with cilia, which 

 exhibit their vibrating and undulating motions, 

 and cause a constant current of water over the 

 surface of the gills, serving here, no doubt, as 

 in analogous instances, at least chiefly for 

 respiration. 



In a paper already referred to,* I mentioned 

 having observed the phenomena in question in 

 the Amphitrite. The animal meant was a com- 

 mon marine tubicolar worm (fig. 300), which 



Fig. 300. 



Amphitrite alveolata. 



A. Dorsal surface, natural size. 



B. Part before a, b, magnified. 



C. A gill magnified. 



D. One still more magnified, to show the spiral 



ridges and cilia. 



appeared to be the same with that figured by 

 Ellis (Corall. plate 36), and described by Cuvier 

 as the Amphitrite a ruche, with which figure it 

 agrees, except that it bears two rows of simple 

 filaments on the back, which, for reasons that 

 will appear, I was led to regard as gills. But 

 if these are really gills, the animal must, it 

 seems, be arranged with the Dorsibranchiata, 

 probably as a Sabella. The currents in this 

 worm proceed forwards along the back, be- 

 tween the rows of gills (as marked in Jig. B), 

 and along the gills themselves (see C), whose 

 points are directed forwards. The conical fila- 

 ment of which each gill consists is marked on 

 one side by ridges (see C, D), crossing it 

 obliquely like segments of a spiral ; and on 

 these ridges as well as on the point of the gill 

 the most conspicuous cilia are placed. The 

 cilia are comparatively large and curved, 

 their points being turned towards the summit 

 of the gili, which figure they retain when their 

 motion is stopped. The gills contain large 



* Edin. Med. and Sur. Jour. vol. xxxiv. 



bloodvessels, which when distended give them 

 a bright red colour. 



The ciliary motion occurs also on what seem 

 to be the branchiae of another tubicolar worm, 

 the name of which is unknown to me; the 

 organs in question are placed at the anterior 

 extremity of the animal, concealed by a pro- 

 fusion of long serpentine tentacula. 



Lastly, Mr. Cheek* observed the ciliary 

 motion in the Sandworm ( Arenicola piscato- 

 rum). It was seen on the inner surface of the 

 internal vesicles, which Sir Everard Home de- 

 scribes as livers. Nothing similar exists on the 

 tufts of filaments which form the gills.f 



8. Mollusca. The ciliary motion prevails 

 very extensively in this division of the animal 

 kingdom. It seems to exist generally in Ihe 

 Gasteropodous and Acephalous Mollusca. 

 There is some uncertainty as to its existence 

 in the Cephalopoda ; I have repeatedly sought 

 for it in that class, but without success. 



It occurs on the surface of the respiratory 

 organs, and often on other surfaces over which 

 the water has to pass in the act of respiration. 

 It also exists within the alimentary canal, at 

 least this has been ascertained in several spe- 

 cies of Gasteropoda and Acephala, and may 

 be presumed of the rest. Moreover, in some 

 of the Gasteropoda, it is very manifest on the 

 horns or feelers, which suggests the possibility 

 of its aiding in these instances in the exercise 

 of the sense of touch or smelling. In all 

 cases the impulsion maintains a determinate 

 direction, which continues the same in parts 

 detached from the animal. In salt-water spe- 

 cies, the action of the cilia and impulsion of 

 the fluid, are instantly stopped by putting the 

 parts into fresh water. 



The ciliary motion also occurs in the embryo 

 of the Mollusca within the egg, which pheno- 

 menon will be considered in the next section. 



A. Gasteropodous Mollusca. Of this class 

 the phenomena have been observed by myself 

 and others in the orders of Nudibranchiata, 

 Cyclobranchiata, Pectinibranchiata, and the 

 aquatic PulmoniJ'era, in one or more species 

 of each. 



a. Nudibranchiuta. In this order, in which 

 the gills are entirely exposed, the currents can 

 be very easily observed. The Doris, a species 

 of which is represented in the adjoining figure 

 (301), may serve as an example. The arbo- 

 rescent gills (, a) are ranged in a circle round 

 the anus, and their stems and branches are 

 covered with cilia. Currents pass over their 

 surface, the general direction being towards 

 the points; small portions detached still ex- 

 cite currents in the same direction, and, if free, 

 move through the water in the opposite one. 

 I have examined three species of Doris, and 



* Edin. Journ. of Nat. and Geog. Science, 

 April, 1831, p. 245. 



f The ciliary motion has also been observed in 

 PlanariiE, on the surface of the body, by Gruit- 

 huisen, (Salzb. Med. Chir. Zeit. 1818, vol. iv.) 

 and by Purkinje and Valentin Gruithuisen also 

 discovered it in the Nais proboscidea, in the pos- 

 terior part of the intestine, (Nov. Act. Acad. Caes. 

 Leop. xi. p. 238.) 



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