Respiration in Invertebrate Animals. 259 



interlamellar water-tubes, these rows of cilia are single on the 

 opposite sides. The excurrent streams are consequently driven by 

 a power which is one-half in amount of that by which the water 

 is propelled through the lamellse into the tubes. No cilia are 

 detectable on the supporting framework between the lamellse. The 

 current raised by the cilia which are distributed on the external 

 surface of the lamella3 tends in the direction of the free border ; 

 that excited by the internally-placed cilia bears towards the 

 proximal border, coinciding with the outlet of the interlamellar 

 tubes. It is a true ciliary current ; but it is reinforced, quick- 

 ened from time to time, by the contractile, voluntary action of 

 the musculo-fibrous parts which constitute the intervectal and 

 interlamellar framework. The cilia which fringe the free border 

 of the gill propel the water in the direction of the mouth, at 

 right angles, consequently, with that raised by those covering 

 the flat surfaces of the gill. It is an alimentary, not a respira- 

 tory current. It is powerful enough to bear on its waves pellets 

 of food for the mouth. The true aerating currents travel along 

 the naked lines between the rows of cilia. These are the 

 ultimate scene of the respiratory process. 



Thus ends an imperfect sketch. It is but a rude outline of 

 a beautiful picture. Much is left to the industry of observers 

 coming after the author. Magnetic thoughts, indicating the 

 pole of truth, have been but hastily projected. May they 

 stimulate others to a truth-admiring repetition of his labours ! 



EXPLANATION OF PLATES VL VII. and VIII. 

 Plate VI. 



Fig. \. Complete view of the double gill of Pholas Candida. The attached 

 border is held upwards as it were, a, b, are the large afferent 

 and efferent trunks (of which there is a set in the border of each 

 lamella) communicating with the parallel vessels, as shown at {n) 

 and {h) ; c, c, c, mark the longitudinal pieces of the intra-tubular 

 framework of solid structures ; k, k, k, show the mode in which 

 these longitudinal pieces form and bound the interlamellar water- 

 tubes {j,j,j) ; d, d, d, are the transverse pieces of the framework 

 on which the lamellse {m, c) of parallel vessels rest, and by which 

 the latter are held apart, and which cross at right angles the axes 

 of the interlamellar water-tubes ; e, f, denote the two series of 

 loops of which the free border of this double gill is composed ; 

 they lie on two distinct planes one above the other ; they are 

 shown as if they were few in number and far between ; they exist 

 in nature in innumerable multitudes and packed with dense close- 

 ness, m, n, represent the course of the bars in the upper lamella ; 

 e, i, that of the lower. The arrows at the base of the gill emerge 

 out of the interlamellar water-passages, g, intervascular ^ferott* 

 connecting pieces. 



Fig. 2. Is a view of a small portion of the entire single or supplementary (sic) 

 gill in Pholas Candida, a, b, afferent and efferent trunks giving and 



17* 



