Respiration in Invertebrate Animals. 261 



lamellae are fused together at the free border, and the intermediate 

 tubes are caecal. 



Fig. 12. A few looped bars from the preceding, magnified, showing the de- 

 licate transverse threads, c, c, c, which cross the branchial bars at 

 distant intervals. They lie on the inside or tubular aspect of the 

 lamellae, and sometimes supersede the intra-tubular framework. 

 The open spaces (d) between these cross threads are the " inter- 

 vectal " water-orifices, a, b, are either vascular bars, or two rigid 

 sides bounding an intermediate membranous channel, which then 

 is the blood-channel. 



Fig. 13. A small portion from the distal edge of the double or upper gill 

 of Cardium. a, b, the upper and lower loops of which this border 

 is composed. The planes of the loops have a vertical position in 

 relation, that is, to the plane of the whole gill, c, the fleshy 

 or membranous structure which unites the loops. It belongs 

 probably to the intra-tubular apparatus, d, d. Although in this 

 double gill, as in all double acephalan gills, the free edge is double, 

 the interlamellar water-tubes /, and arrows g, are single. The 

 double row of loops runs into one at a little distance from the 

 margin, in order to form one system of tubes. This union of the 

 bars is shown at i and e. h, indicates the fleshy structures, — a part 

 of the intra-tubular cross bars by which the lobes or " pectina- 

 tions " of the gill are held together. 



Fig. 14. A portion of the free margin of the inferior or single gill of Car- 

 dium. This figure exhibits perfectly the manner in which the 

 limbs (b) — (which, arranged in a linear series, form the upper wall 

 of the tube (d), or the upper lamella of the gill) — of the same 

 system of loops pass, by looping vertically, as shown at a, into 

 those of the lower wall of the tubes, or, which is the same thing, 

 into the lower lamella of the gill. 



Fig. 15. Shows the alternate mode by which the membranous blood- 

 channels (b, b, b) are formed b}^ the solid bars (a, a, a, a) of con- 

 tiguous, but distinct loops. In such a case, which is the normal 

 type in the branchiae of all Tunicata, the cilia are disposed in 

 lines only on that side of each bar which is nearest to the water- 

 fissures (c). These fissures, in such examples, are not crossed by 

 transverse connecting threads. 



Fig. 16. Represents one of the lobes, or tubular pectinations, from the 

 single gill of Cardium, cut longitudinally, illustrating the mode 

 in which the water-tubes are formed. 



Plate VIII. 



Fig. 17- The "bars" from the gill of Mytilus, followed throughout the 

 whole of their minute anatomy, a, b, are the afferent and efferent 

 blood-trunks, running along the attached margin of the upper 

 lamella A. This border of the gill in Mytilus is represented as 

 including two trunks, on the theory, as yet not quite proved, 

 that the two edges, c and c, would then be the beginning of the 

 blood-channel along the blunt edge, and a, a, a, would mark the 

 termination of that, travelling along the acute margin of the same 

 penknife-shaped process. j,j, are the corresponding trunks at 

 the proximal border of the inferior lamella ; e, e, mark the con- 

 tinuous membrane by which, in this gill, the bars are tied together 

 at the proximal border, so that no water can pass between the bars ; 

 f,f,f,f, are open orifices between the fleshy nodules, o, o. o, o, 

 by which the aerating water enters from without into the inter- 



