Respiration in Invertebrate Animals, 89 



sustaining a double series of parallel blood-channels. The ana- 

 tomy of the folded portion of the leaf may be better understood 

 on a tranverse section (such section is represented at fig. B). 

 It will be seen that the duplications of the surface are the same 

 on both sides (f,f), and that the opposite folds are formed upon 

 the same transverse axis. 



The next problem to be solved in the analysis of this most 

 beautiful mechanism relates to the disposition of the blood- 

 vessels [h, h). This question could never be determined by in- 

 jections, however fine or successful. The practised eye, reading 

 the same structure throughout an extended series of varieties 

 and modifications, may infallibly decipher the blood-system 

 even of these subtle and delicate laminae. The leaf tapers away 

 into a slender point at the extreme membranous end [j). If 

 the same blood-channel, or the same drop of blood, travelled 

 from the dorsal border {a, a) across the entire length of the 

 laminae, as far as the tapering point (j), it is evident that such 

 a portion of blood would undergo an excessive degree of aeration, 

 while that which traversed the space at either apex [i, h) would 

 fall short of the required amount of oxygenation. This incon- 

 venience is obviated by giving to the vessels ?i generally oblique 

 direction across the plane of the lamina {d, b). The vessels as 

 they traverse the folds (c, c and b, b) maintain the same character 

 and direction as they exhibit on the smooth portions. This 

 fact proves that these folds are really none other than duplica- 

 tions of the surface. They serve, notwithstanding, to multiply 

 the active superficies of the little organ, and the vessels which 

 they carry, although unaltered in direction, are smaller in 

 diameter and more closely arranged. The ciliary action over 

 these portions is also more active and vigorous than on other 

 parts. The vessels are most distinct in outline and disposition 

 about the central region of the leaf [h, h, fig. 4 d). That the 

 channels are conduits, distinct and individualized, separately 

 walled, running side by side, and seldom inosculating, may be 

 unquestionably proved by the steady examination of this part. 

 That the channels loop around the free edge (figs. 5 & 6) to gain 

 the other side, along which they return, the observer may con- 

 vince himself by focusing the microscope at this border. The ap- 

 pearance is then such as is shown in hg. 5, and fig. 9 b. The 

 cilia of the flat surface are seated on a pavement epithelium, those 

 of the borders stand out like filaments from a larger description 

 of cell (fig. 8). 



The author has proved, that in all the species of the genus 

 Littoinna discoverable on the coast around Swansea, the branchial 

 laminae are constructed on the model (fig. 3) of that just de- 

 scribed. The duplications are not of the same precise size and 

 character in all ; but in all they exist. He would propose this 



