CILIA. 



623 



motion of the other set consists in a succession 

 of undulations, which proceed in a uniform 

 manner along the sides of the bar from one 

 end to the other. It might be very easily 

 mistaken for the circulation of globules of a 

 fluid within a canal, more especially as the 

 course of the undulations is different on the 

 two sides of the bar, being directed on one 

 side towards the edge of the gill, and on the 

 other towards the base. But besides that the 

 undulations continue for some time in small 

 pieces cut off from the gill, which is incon- 

 sistent with the progression of fluid in a canal, 

 the cilia are easily distinguished when the un- 

 dulatory motion becomes languid. When it 

 has entirely ceased, they remain in contact with 

 each other, so as to present the appearance of 

 a membrane, (d, d, fg. F.) Besides the two 

 rows of cilia just described on each side of the 

 bars, others are placed in a less regular manner 

 on their external and internal borders. The in- 

 ternal (h, Jig. C) are exceedingly small ; they 

 extend upon the cross slips, (i,Jig- C). Those 

 on the external borders are very numerous and 

 thick-set, and of considerable size, especially 

 on the extremity of the bar at the edge of the 

 gill (c, Jig. C) ; their points are directed to- 

 wards the edge of the gill. It is probably by 

 the agency of these last-mentioned cilia that 

 the particles of food or other foreign matter 

 are conveyed along the surface of the gill to 

 its edge, and then onwards to the mouth, 

 while the others may serve principally to force 

 the water through the interstices of the bars 

 into the space inclosed between the layers, 

 and from thence out at the excretory orifice. 



As in other instances, detached portions of 

 the ciliated parts excite currents in the same 

 direction as before their separation, or swim 

 through the water in the opposite direction. 

 It is very remarkable that when the parts are 

 immersed in fresh water, the currents and mo- 

 tion of the cilia are almost instantaneously 

 stopped. 



The ciliary motion is equally apparent on 

 the respiratory organs of the Oyster, River-mus- 

 sel, and other bivalve Mollusca which have 

 been submitted to examination. Purkinje and 

 Valentin pointed out its existence also in the 

 alimentary canal of the River-mussel, which 

 observation I have confirmed, and I have found 

 the same to be true of the Sea-mussel. The 

 impulsion appeared to me in both instances to 

 be chiefly directed onwards, that is, towards 

 the anus. 



c. Tunicuta (Ascidue). In the paper pre- 

 viously referred to, I stated that I had not been 

 able to perceive the ciliary motion in the Ascidia, 

 but added that the observation seemed inconclu- 

 sive, as the specimens examined had been some 

 time out of the water. Since then I have seen 

 the phenomena as distinctly in the Ascidise as 

 in other Mollusca. The observations were made 

 on a common species found adhering to rocks 

 in the Frith of Forth at low water-mark, and 

 as far as they go they agree with those lately 

 made by Mr. Lister,* on a small aggregated 



* Phil. Trans. 1834, p. 378. 



species, the substance of which being nearly 

 transparent enabled him to trace the currents 

 more completely. For this reason it seems 

 preferable to borrow his description. 



The annexed figures (A and B) represent 



Fig. 306. 



-k 



i 



one of these Ascidiae on 

 its peduncle, with the 

 opening of the mouth (g) 

 and the funnel (_/'') in 

 front. The outer covering 

 is a tough coat (<v), lined 

 internally with a soft sub- 

 stance or mantle (t>). A 

 great part of the interior 

 is occupied with the 

 branchial sac (c), whose 

 cavity terminates upwards 

 at the oral opening, and 

 is closed at the bottom. 

 It is united to the enve- 

 lope or to the mantle 

 above and behind ; the 

 juncture (e,e,) beginning 

 in front of the oral open- 

 ing, extends backwards 

 on each side of it, and then downwards along 

 the middle of the back (a, Jig. A.) A vacant 

 space (,/',./,) is left between the sac and mantle 

 at the sides and front, which ends in the 

 opening of the funnel. The sac opens infe- 

 riorly into the oesophagus (/;), which leads to 

 the stomach (i), the intestine passing forwards 

 and opening by the vent (/c) into the funnel. 

 On its sides and front the branchial sac is per- 

 forated by four rows of narrow vertical slits or 

 spiracles (in, m), and through these the water, 

 which flows constantly in at the mouth when 

 its orifice is open, appears to be conveyed to 

 the vacant space (/') between the sac and 

 mantle, and it then escapes at the funnel. 

 The sac seems extremely thin between the 

 spiracles, but their edges are thickened, and 

 they are lined with closely set cilia, which, by 

 their motion, cause the current of water. When 

 they are in full activity, the effect upon the eye 

 is that of delicately toothed oval wheels, re- 

 volving continually in a direction ascending on 



