616 



CILIA. 



In the inside of the body the membranous 

 stomach (g) occupies the middle part, and 

 from it a pair of lobed coeca (h, hj (and i, i, cut 

 short) pass into each ray. Within the rays also 

 we find inferiorly the rows of vesicles (k, k) 

 which form part of the feet (I, I), and the 

 ovaries. All the rays communicate through 

 the middle part, and the whole inside is lined 

 by a transparent membrane (n, n)j which, 

 like a sort of peritoneum, covers the stomach 

 and cceca, attaches each of the creca by a me- 

 sentery (o f o) to the roof of the ray, lines the 

 fibrous parietes of the body, and is probably 

 reflected over the vesicles of the feet and the 

 ovaries. Each mesentery encloses a space 

 (i fis- B) between its sides, which opens into 

 the general cavity at the root of the cceca. The 

 lining membrane passes into the perforated 

 pits ( e)j by which the tubes (c) communicate 

 with the cavity, and sends prolongations through 

 the perforations into the tubes lining them to 

 their points. The space (s, *, jig. B) lined by 

 this membrane contains sea-water, which is 

 generally described as entering and issuing by 

 the respiratory tubes.* 



I find the ciliary motion in four situations, 

 namely, 1. on the external surface; .2. within 

 the cavity of the body, or in the space ^(s) 

 between its parietes and the viscera ; 3. within 

 the stomach and coeca; 4. within the feet. 

 In all these situations moving cilia are visible 

 with the microscope on the respective surfaces; 

 they are every where comparatively small, in 

 some parts excessively so. Though I have not 

 traced them over the entire extent of each sur- 

 face, I have no doubt they exist at every point 

 where currents are produced. 



1. On the external surface. The ciliary mo- 

 tion as indicated by the application of pow- 

 dered charcoal, occurs over nearly its entire 

 extent, but with different degrees of intensity. 

 The strongest currents pass along the outer 

 surface of the tubes from the base to the point, 

 as at c ; they are also pretty strong on the 

 claw-like processes (V) and intermediate skin; 

 on the feet they are evident but less vigorous. 



2. Within the body the currents take place 

 on the lining membrane and its reflections. A 

 longitudinal current runs along the roof, and 

 another along the floor of each ray, forwards 

 or towards its point : (see the arrows in jig. A.) 

 These advancing currents are confined to the 

 median line and its immediate vicinity; two 

 retiring currents (r, r,) run backwards (one on 

 each side) at the place where the sides join the 

 floor of the ray. Two longitudinal currents also 

 exist on each of the coeca, an advancing one 

 (h') on the inferior surface, and a retiring one 

 superiorly (h, h, fig. A) in the space (o,jig. B) 

 inclosed within the mesentery, which, as already 

 mentioned, opens into the general cavity. The 

 longitudinal currents, except those within the 

 mesentery, are, if for the sake of explanation 



* Without denying this mode of entrance, 1 

 may yet mention, that though I have often seen 

 the animal slowly distending itself with water, and 

 again partially emptying itself, 1 could never per- 

 ceive the fluid entering or issuing at the, orifices 

 described. 



we may so express it, connected by others 

 which run vertically and transversely on the 

 coeca and on the roof and sides of the cavity, 

 (see the arrows in Jig. B;) on the vesicles of the 

 feet the course of these cross currents is varied 

 by the curved surfaces. As the lining membrane 

 of the cavity extends into the respiratory tubes, 

 so currents exist within these likewise, as at t, 

 jig. C. This is proved by injecting turbid 

 fluid into the ray, when particles are seen 

 moving within the tubes ; and if a few of the 

 tubes with a portion of the skin be cut off and 

 placed under the microscope, the fluid which 

 will still be retained by some of them may be 

 seen to be in motion, the floating particles 

 moving from the base to the point and back 

 again, as in the arms of the Actiniae. 



3. The motion is very distinct on the inner 

 surface of the stomach and coeca ; the currents 

 within the coeca follow the same direction as on 

 their external surface, that is, an advancing 

 current runs inferiorly from the root to the 

 point and a returning one superiorly ; and at 

 the sides currents run upwards, following the 

 ridges or folds of the internal membrane which 

 result from the lobulated structure of the 

 cceca. 



4. The ciliary motion exists distinctly within 

 the feet, though the cilia are very small ; these 

 became visible on viewing the edge of a folded 

 portion with Wollaston's doublet of one-thirty- 

 fifth of an inch focus. 



The currents described, as far as I have been 

 able to perceive, preserve always the same 

 determinate direction. Even when portions of 

 the ciliated surface are detached, the motion 

 on them continues, and its direction is the 

 same as before their separation. 



As to the use of these motions, it is most 

 probably connected chiefly with respiration; 

 and if such be the case, it would show that in 

 this animal a great extent and variety of parts 

 are concerned in that function. The ciliary 

 motion on the inner surface of the stomach and 

 coeca is probably subservient also to the process 

 of digestion. It is conceivable that by means 

 of this provision the dissolved or digested food 

 might be introduced into the coeca, and spread 

 over their internal surface, there to be duly 

 mixed with secreted fluidsjand subjected to 

 the process of absorption ; the returning cur- 

 rent serving to bring back the residue, or to 

 convey secreted fluids into the stomach. Or, 

 considered as subservient to respiration, the cili- 

 ary motion, in diffusing the digested food over 

 the internal surface of the coeca, may at the 

 same time expose it to the respiratory influence 

 of the water on their outside. 



These phenomena in the Asterias seem not to 

 have been previously noticed. Tiedemann,* it 

 is true, had observed an eddying motion of the 

 water in the vicinity of the respiratory tubes 

 while the animal was slowly distending or 

 emptying itself, but he conceived it to be 

 nothing more than the commotion necessarily 

 produced by the passage of the water through 

 the tubes. There can be little doubt that the 



* Anat. der Rohren Holothurie, etc. p. 40. 



