LAMELLIBRANCHIATA. 15 



aii', and the inner surface of which is thrown into a number of 

 complicated folds, so as to expose a large surface for the respiratory 

 blood-vessels; it communicates by an opening with the extern,! 1 

 medium. The pulmonary and branchial cavities are, therefore, mor- 

 phologically equivalent. 



Reproduction is always sexual. The hermaphrodite condition, 

 on the whole, preponderates ; nevertheless, not only many marine 

 Gastropods, but al>o most Lamellibranchs and all Cephalopods are 

 dioecious. 



Development usually begins with a total segmentation, which is 

 followed by the formation of a blastoderm surrounding the hinder 

 part of the yolk or the whole yolk. The just hatched young often 

 pass through a complicated metamorphosis, and possess an anterior 

 cutaneous expansion bordered with cilia the velum which func- 

 tions as a locomotory organ. In form, disposition of cilia and 

 organisation, many molluscaii larva? permit of a closer comparison 

 with Loven's worm larva. 



By far the majority of the Mollusca are aquatic animals, especiallv 

 marine ; only a few live on land, and these always seek damp 

 localities. When we consider the extraordinarily wide distribution 

 of the Mollusca in past times, the importance of their fossil remains 

 for the determination of the age of the sedimentary formations 

 becomes intelligible. 



Class 1. LAMELLIBRANCHIATA.* 



Laterally compressed Mollusca without separated head, with bilobed 

 mantle and bivalve shell, composed of a riyht and left half and 

 connected by a dorsally -placed ligament ; with large gill plates ; sexes 

 usually separate. 



The Lamellibranchs were formerly united with the Brachiopoda 

 as Conchifera. Like the latter, they lack a differentiated cephalic 

 region, and possess a large and usually bilobed mantle and a 

 bivalve shell. Nevertheless, the structural differences between these 



* G. Cuvier, " L' histoire et 1'anatomie des Mollusques." Paris, IS I 7. 



Bojanus, " Ueber die Athem- und Kreislaufswerkzeuge dor z\veischaligcn 

 Muscheln." Jv/.v, 1817, 1820, 1827. 



S. Loven, A". Vet. Alui'l . Handlgr. Stockholm, 1848. Translated in the 

 Arch, fur Natwgescli., 1849. 



Lacaze-Duthiers, Ann. tics Sc. Nat.. 1854 181. 



H. and A. Adams, "The Genera of the lleeent Mollusca." London, 1853- 

 1858. 



T . Reeve. " Conchologia icouica." London, 1846-1858. 



