CILIA. 



621 



the inner surface of the mantle, where it forms 

 the roof of the branchial cavity. Their situ- 

 ation and direction are indicated in the figures 

 by the arrows. B is an enlarged view of a 

 few laminae from the larger series, h the at- 

 tached border, i point, m left, and n right 

 border. Currents pass between these laminae 

 along the surface and border of each, as 

 shewn in B; C is a magnified view of the 

 laminae of the smaller set, on which the di- 



rection of the currents is marked ; the direc- 

 tion on other parts will be understood by re- 

 ferring to figure A. 



The ciliary motion is very manifest within 

 the alimentary canal, in the gullet, stomach, 

 and intestine; the direction of impulsion is 

 from the mouth towards the anus. 



The ciliary motion has been observed by 

 myself and others in the Paludina vivipara, 

 a fresh-water snail belonging to this order, 



Fig. 305. A 



o- 



c. 



-a 



in which also Purkinje and Valentin state 

 that they observed it within the alimentary 

 canal; and Gruithuisen* has described 

 the phenomenon as seen on the branchiae 

 of another fresh-water snail, which he 

 names Valvata branchiata. He saw moving 

 cilia, which caused an incessant agitation 

 in the water ; but he does not state whether 

 the motion followed any constant direc- 

 tion, although we may infer that this was 

 the case. He rightly attributed to these 

 motions a respiratory function, but seems 

 not to have observed that similar pheno- 

 mena existed in other Mollusca. 



d. Pulmonifera. The ciliary motion is 

 not confined to those Mollusca which 

 breathe by gills, for it occurs also in the 

 Lymnaea and Planorbis, which, though 

 they live in water, breathe air by a pul- 

 monary sac. In these instances the impulsion of the water 

 takes place on the surface of the tentacula, which is covered 

 with cilia. If these parts are to be regarded as organs of 

 sensation alone, the ciliary motion observed upon them, 

 as well as that which occurs on the tentacula of bran- 

 chiferous species, must be considered as connected with 

 the function of sensation ; but the tentacula, which in the 

 Lymnaea are broad vascular laminae, might be conceived 

 also to perform the office of accessory organs of respiration, 

 in which case the pulmoniferous Mollusca here mentioned 

 would possess organs both of aerial and aquatic respiration. 

 In the Lymnaea the motion has also been observed by 

 Purkinje and Valentin within the alimentary canal. 



Conchiferous AcephfJa. The motion in question has 



B. 



d' 



Mytilus Edulis. 



F. Portion of a bar of the gill, with 

 the cilia, highly magnified. 



been found in several bivalve Mollusca, both of salt and 

 fresh water, and there can be little doubt that it exists in all. 

 The common Sea-mussel (fig. 305) will serve as an 

 example of the class. It will be recollected that the 

 gills of this animal (fig. A, c, c', d } ) have the form of 



* Nova Acta Acad. Caes. Lcop. x. p. 437. 



