Respiration in Invertebrate Animals. 55 



act no part either in the alimentary or respiratory process. It 

 cannot reach the mouth without permeating the branchial fora- 

 mina. It can do this only by overcoming the opposing force 

 exerted by the branchial cilia. If the water which occasionally 

 enters at the exhalent siphon were normally designed for respi- 

 ration, like that flowing in at the inhalent siphon, as supposed 

 by Mr. Clark, the former could not reach the branchial lamellae 

 without meeting and mingling with the counter impure current 

 driven by cilia, coming from the branchiae. The principle is ab- 

 solute in the mechanism of the respiratory process in all Tuni- 

 cate and Lamellibranchiate mollusks, that the aerating element 

 must pass through the gill. The function of breathing cannot be 

 accomplished if the water merely passes in a stream whose 

 direction is parallel with the planes of the gill-laminae. The 

 ultimate and true respiratory currents are those which permeate 

 the laminae at right angles to their planes. It results of logical 

 necessity, therefore, that if the water did not traverse the bran- 

 chial partition (PI. II. fig. 12, g) which divides the extra- from 

 the intra-branchial chambers, the great and imperious function 

 of respiration could not be accomplished. 



The preceding postulates, supported only by theoretical rea- 

 soning, will on a future occasion be rendered incontrovertible 

 by the demonstrations of minute anatomy. 



EXPLANATION OF PLATES I. and II. 

 Plate I. 



Fig. 1. Clavellina lepadiformis (after Milne-Edwards): a, mouth and in- 

 halent funnel ; b, large horizontal blood-channels, running regu- 

 larly between the vertical ones q ; p, the intervascular stigmata 

 through which the water passes from the branchial chamber into 

 the cloacal cavity (o), whose outlet is at (h) the exhalent or anal 

 siphon ; /, rectum terminating in the cloaca ; n, mantle ; m, test, 

 the intermediate space being rilled with blood. 



Fig. 2. Pyrosoma giganteum (after Milne-Edwards) : a, mouth ; b, bran- 

 chial sac ; c, oesophagus ; d, stomach; e, intestine ;/, liver ; g, anus ; 

 k, ovarv ; I, ganglion. 



Fig. 3. Perophora Listen : a, branchial siphon ; b, tentacles at its base ; 



c, large blood-channels in which the blood-corpuscles are seen ; 



d, branchial, e, cloacal chamber ; d, exhalent siphon ; e, space, 

 tunnelled with spacious blood-channels, between the test and 

 mantle. 



Fig. 4. Plan of vessels in the branchial band of Salpa maxima : a, large 

 vessels constituting a separate layer ; b, smaller ; c, csecal pro- 

 cesses from the vessels and rilled with blood, and communicating 

 with their internal channel ; d, d, water meshes : tig. 4 a, a sepa- 

 rate bar, showing cilia, and bore. 



Fig. 5., One of the branchial folds of Cynthia microcosmus : a, c, water 

 meshes between the blood-channel ; b, d, the attached borders ; 



e, free border ; d, transverse blood-channels. 



Fig. 6. Branchial system of Pholas dactylus : a, intra-branchial or exhalent 



