THE LAMELLIBRANCHIA 233 



In other cases the differentiation into two branches folded one over 

 the other disappears through the reduction of the antero-posterior 

 branch ; the kidney is then a simple sac into which the reno- 

 pericardial duct opens. This may be seen in some Filibranchia. 



The two kidneys do not communicate with one another in 

 Solenomya and in many Filibranchia, but a communication between 

 their anterior ends is already present in Yoldia and Leda, and 

 a communication, often of considerable extent, between their 

 posterior ends is found in the majority of the more specialised 

 forms, e.g. Modiolarca, Lasaea, Donax, Tapes, Pseudokellya, and 

 especially in the Myacea, Pholadidae, and Anati- 

 nacea. The renal organs are excessively ramified 

 and extend over the whole surface of the visceral 

 mass in Ostraea, and both in this genus and in 

 Pholas they surround the posterior adductor 

 muscle. The kidneys similarly extend very far 

 forward in Mytilus and in the majority of the 

 Anatinacea, penetrating into the mantle itself 

 in Lyonsiella. Finally, in the Septibranchia the FIO. 213. 



kidneys are almost wholly immersed in the Left kidney of 



11-1 /-[TV mi VTTA " l V a medlterranea, ventral 



pallial SinUS (Fig. 211, XIV). aspect. I, posterior part 



In the most archaic Lamellibranchia the JflSfft 

 renal secretion is passed out of the body in pericardial orifice ; iv, 



v -J f u L i.u ^ c c duct of the gonad. 



a liquid form, but in others in the form of 



solid concretions, exhibiting concentric layers of growth, and in 



normal conditions containing only urea. 



The excretory function is carried out by the pericardial glands 

 as well as by the kidneys. These glands are differentiations of 

 the epithelial wall of the pericardium, and may be localised on 

 and impart a brownish tint to the auricles, as in many Filibranchia 

 (Arcidae, Mytilidae, Pectinidae) and the Ostraeacea, or they may 

 be near the auricles as in the Aviculidae. In this condition 

 they are less well developed in the more specialised groups, but 

 they reappear in Pholas and Saxicava. The glands frequently 

 exist in the form of a glandular lining of the anterior part of 

 the pericardium or of the glandular diverticula which pass from the 

 pericardium into the mantle (Fig. 212, pe.g). This arrangement 

 may be seen in the Unionidae the diverticula constituting the 

 " organ of Keber " in certain Lucinidae, Veneridae (Fig. 202, pe.g), 

 Tellinidae, Solen, Pholas, and Aspergillum. In the last-named 

 genus these diverticula used to be called pericardial veins at a 

 time when the pericardium was thought to be a blood sinus. The 

 pericardial glands excrete hippuric acid, which is poured into the 

 pericardium, and from thence passes into the kidneys. 



4. Nervous System and Sense-Organs. The different pairs of nerve- 

 centres are always placed at some distance from one another in the 



