118 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



rods, two of which run vertically within each ridge. Beneath 

 the epithelium the gills are composed of loose connective tissue, 

 everywhere permeated by blood-containing lacunae. The inter- 

 lamellar junctions are traversed by more definite blood-channels. 

 Respiration is said to be effected more by the mantle than the 

 gills, which are elaborate current-producing organs, and, as usual, 

 essentially consists of gaseous interchange between the blood and 

 the surrounding medium, oxygen being taken up and carbon 

 dioxide eliminated. Water entering the branchial chamber by 

 the inhalent aperture partly passes to the mouth and partly into 

 the water-tubes of the gills through the numerous small apertures 

 in the lamellae. It then runs upwards into the supra-branchial 

 passages, and backwards through the exhalent aperture to the 

 exterior. All the currents are the result of ciliary action. 



6. Excretory Organs (Figs. 32 and 34). A pair of kidneys 

 (organs of Bojanus) are present, each of which is essentially a 

 tube folded upon itself, and communicating on the one hand with 

 the pericardial cavity, which appears to represent a much-reduced 

 coelom, and on the other with the exterior by the renal opening 

 noted above. The tube is divided into a thick-walled glandular 

 part, the lining of which is raised into numerous ridges, and a 

 thin-walled ureter (non-glandular part). The vena cava lies 

 between the two ureters, which open in front by the renal 

 openings, and communicate posteriorly with the glandular parts 

 which underlie them. The glandular part is broadest behind, 

 where it abuts upon the posterior adductor, and narrows in front 

 where it opens into the floor of the pericardium. The kidneys 

 should perhaps be regarded as nephridia, like the excretory organs 

 of earthworm, since they are excretory tubes opening out of a 

 coelomic body-cavity. 



The glandular parts of the kidneys are lined by glandular 

 epithelium, the cells of which are granular, and often contain 

 concretions, in which guanin (C 5 H 5 N 5 0), uric acid (C 5 H 4 N 4 3 ), 

 and urea (CH 4 lSr 2 0), have been stated to occur. The organs of 

 Keber, consisting of glandular tubules, and situated one on each 

 side of the pericardium, into which they have been said to open, 

 may also have an excretory function. 



7. Eeproductive Organs. The sexes are distinct, and the 

 female is somewhat thicker from side to side. The genital 

 glands (spermaries or ovaries, as the case may be) are much- 



