Elementary text-book of zoology. Special part: Mollusca to man / by C. Claus ; translated and edited by Adam Sedgwick ; with the assistance of F.G. Heathcote.

  • Claus, Carl, 1835-1899.
Date:
1897
Catalogue details

Licence: Public Domain Mark

Credit: Elementary text-book of zoology. Special part: Mollusca to man / by C. Claus ; translated and edited by Adam Sedgwick ; with the assistance of F.G. Heathcote. Source: Wellcome Collection.

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    air, and has its inner surface thrown into a number of compli- cated folds, so as to expose a large surface for the respiratory blood- vessels ; it communicates by an opening with the external medium. The pulmonary and branchial cavities are, therefore, morphologically equivalent. Reproduction is always sexual. The hermaphrodite condition, on the whole, preponderates; nevertheless, not only many marine Gastropods, but also 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 molluscan larvae permit of a closer comparison with Loven’s worm larva. By far the majority of the Mollusca are aquatic animals, especially 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 right and left half and connected by a dor sally 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 * Gf. Cuvier, “L’ histoire et l’anatomie des Mollusques.” Paris, 1817. Bojanus, “Ueber die Athem- und Kreislaufswerkzeuge der zweisckaligen Muscheln.” Isis, 1817, 1820, 1827. S. Lov6n. IC. Vet. Altad. Handlgr. Stockholm, 1848. Translated in the Arch, für Natur gesell., 1849. Lacaze-Duthiers, Ann. des Sc. Kat., 1854—1861. H. and A. Adams, “The Genera of the Recent Mollusca.” London 1853- 1858. L. Reeve, “ Conchologia iconica.” London, 1S46-185S.