376 COMPARATIVE ANATOMY. 



over o tli or functions also. Thus they may have relations to the 

 generative organs, as is clearly the case in some of the Lamellibran- 

 chiata. There is good reason also for thinking that the efferent 

 ducts for the generative products in the Cephalopoda are derived 

 from these excretory organs. They have not, therefore, any exclusive 

 relation to excretion. When they are excretory in function, the walls 

 of the canals, which are otherwise simple, undergo a certain amount 

 of metamorphosis, and may then be seen to have a glandular 

 structure. In these cases they may be regarded as " kidneys," on 

 account of the chemical constitution of their products. In this case, 

 if examined under the microscope, they are seen to be provided with 

 secreting cells, the contents of which are formed by granular or con- 

 centrically striated concretions, similar to those which are of such 

 importance in the renal secretions of other groups of animals. 



Where the internal orifice has been observed, it has been seen to 

 lead into the pericardial sinus, through the wall of which the duct 

 passes. If it be true that the excretory organ is derived from a 

 looped canal, it is highly probable that the wall of this pericardial 

 sinus is derived from a dissepiment, such as those which carry the 

 openings of the looped canals in the Annelides. Many facts, however, 

 are wanting to confirm this supposition, and particularly those which 

 would explain how the change in the position of this dissepiment 

 has been brought about. 



§ 290. 



In the Lamellibranchiata the excretory organ is known as the 

 Organ of Bojanus; it is always paired, although sometimes it is 

 fused into one mass along the middle line ; it lies on the dorsal side 

 of the body, close to the base of the gills. Its substance is made up 

 of a yellowish or brownish coloured spongy tissue, the interspaces 

 in which often run together, and generally form a large central 

 cavity. From this cavity a pore, on either side, leads into the peri- 

 cardium, while another leads into the efferent duct. The latter either 

 lies close to the genital pore, or is confluent with it, or, lastly, the 

 generative organs open into the organ of Bojanus, so that the 

 generative products are passed out to the exterior through it (Pecten, 

 Lima, Spondylus). Area and Pinna have the efferent ducts united; 

 Cardium, Chama, Mactra, Pectunculus, Anodonta, Unio, etc., have 

 the orifices of the excretory and generative organs separate. The 

 walls, which rise up in folds, or the meshwork-like tissue of the 

 organ, is thickly invested with secreting cells, which secrete the 

 already-mentioned concretions, in many of which the characteristic 

 excretion — uric acid — has, of course, not yet been observed. As 

 to its relations to the vascular system, see p. 370. 



The Scaphopoda resemble the Lamellibranchiata in the possession 

 of a paired excretory organ. 



§ 291. 



In the Gastropoda the excretory organ varies still more in 

 character. A paired excretory organ — the predecessor of the per- 



