CHAPTER 13 Phylum Mollusca olluscs include some of the best-known invertebrates; almost everyone is familiar with snails, clams, slugs, squids, and octopuses. Molluscan shells have been popular since ancient times, and some cultures still M use them as tools, containers, musical devices, money, fetishes, reli- gious symbols, ornaments, and decorations and art objects. Evidence of histori- cal use and knowledge of molluscs is seen in ancient texts and hieroglyphics, on coins, in tribal customs, and in archaeological sites and aboriginal kitchen middens or shell mounds. Royal or Tyrian purple of ancient Greece and Rome, and even Biblical blue (Num. 15:38), were molluscan pigments extracted from certain marine snails.1 Many aboriginal groups have for millenia relied on mol- luscs for a substantial portion of their diet and for use as tools. Today, coastal nations annually har- vest millions of tons of molluscs commercially for Classification of The Animal food. Kingdom (Metazoa) There are approximately 80,000 described, liv- Non-Bilateria* Lophophorata ing mollusc species and about the same number of (a.k.a. the diploblasts) PHYLUM PHORONIDA described fossil species. However, many species PHYLUM PORIFERA PHYLUM BRYOZOA still await names and descriptions, especially those PHYLUM PLACOZOA PHYLUM BRACHIOPODA from poorly studied regions and time periods, and PHYLUM CNIDARIA ECDYSOZOA it has been estimated that only about half of the liv- PHYLUM CTENOPHORA Nematoida ing molluscs have so far been described. In addi- PHYLUM NEMATODA Bilateria PHYLUM NEMATOMORPHA tion to three familiar molluscan classes comprising (a.k.a. the triploblasts) Scalidophora the clams (Bivalvia), snails and slugs (Gastropoda), PHYLUM XENACOELOMORPHA PHYLUM KINORHYNCHA and squids and octopuses (Cephalopoda), five Protostomia PHYLUM PRIAPULA other extant classes exist: chitons (Polyplacophora), PHYLUM CHAETOGNATHA PHYLUM LORICIFERA tusk shells (Scaphopoda), Neopilina and its kin SPIRALIA Panarthropoda (Monoplacophora), and the vermiform sclerite- PHYLUM PLATYHELMINTHES PHYLUM TARDIGRADA PHYLUM GASTROTRICHA PHYLUM ONYCHOPHORA bearing aplacophoran classes—Caudofoveata PHYLUM RHOMBOZOA PHYLUM ARTHROPODA (or Chaetodermomorpha) and Solenogastres (or PHYLUM ORTHONECTIDA SUBPHYLUM CRUSTACEA* Neomeniomorpha). Although members of these PHYLUM NEMERTEA SUBPHYLUM HEXAPODA PHYLUM MOLLUSCA eight classes differ enormously in superficial ap- SUBPHYLUM MYRIAPODA pearance, there is a suite of characters that diag- PHYLUM ANNELIDA SUBPHYLUM CHELICERATA PHYLUM ENTOPROCTA Deuterostomia nose their fundamental body plan (Box 13A). PHYLUM CYCLIOPHORA PHYLUM ECHINODERMATA Gnathifera PHYLUM HEMICHORDATA This chapter has been revised by Richard C. Brusca, David R. PHYLUM GNATHOSTOMULIDA PHYLUM CHORDATA PHYLUM MICROGNATHOZOA Lindberg, and Winston F. Ponder 1 PHYLUM ROTIFERA *Paraphyletic group Archaeological sites in Israel reveal the probable use of two muricid snails (Murex brandaris and Trunculariopsis trunculus) as sources of the Royal purple dye. B&B_3E Chapter 13.indd 453 12/18/15 3:01 PM 08/28/2016 - RS0000000000000000000000125283 - Invertebrates 454 Chapter Thirteen BOX 13A Characteristics of the Taxonomic History and Phylum Mollusca 1. Bilaterally symmetrical (or secondarily asymmetri- Classification cal), unsegmented, coelomate protostomes Molluscs carry the burden of a very long and convolut- 2. Coelom limited to small spaces in nephridia, heart ed taxonomic history, in which hundreds of names for and gonads various taxa have come and gone. Aristotle recognized 3. Principal body cavity is a hemocoel (open circula- molluscs, dividing them into Malachia (the cephalo- tory system) pods) and Ostrachodermata (the shelled forms), the lat- 4. Viscera concentrated dorsally as a “visceral mass” ter being divided into univalves and bivalves. Joannes 5. Body covered by a cuticle-covered epidermal sheet Jonston (or Jonstonus) created the name Mollusca2 in of skin, the mantle 1650 for the cephalopods and barnacles, but this name 6. Mantle with shell glands that secrete calcareous was not accepted until it was resurrected and redefined epidermal sclerites, shell plates, or shells by Linnaeus nearly a hundred years later. Linnaeus’s 7. Mantle overhangs and forms a cavity (the mantle Mollusca included cephalopods, slugs, and pteropods, cavity) in which are housed the ctenidia, osphradia, as well as tunicates, anemones, medusae, echinoderms, nephridiopores, gonopores, and anus and polychaetes—but included chitons, bivalves, uni- 8. Heart situated in a pericardial chamber and com- valves, nautiloids, barnacles, and the serpulid poly- posed of a single ventricle and one or more sepa- chaetes (which secrete calcareous tubes) in another rate atria group, Testacea. In 1795 Georges Cuvier published a 9. Typically with large, well-defined muscular foot, revised classification of the Mollusca that was the first often with a flattened creeping sole to approximate modern views. Henri de Blainville 10. Buccal region provided with a radula and muscular (1825) altered the name Mollusca to Malacozoa, which odontophore won little favor but survives in the terms malacology, 11. Complete (through) gut, with marked regional spe- malacologist, etc. cialization, including large digestive glands Much of the nineteenth century passed before the 12. With large, complex metanephridial “kidneys” phylum was purged of all extraneous groups. In the 13. Cleavage spiral and embryogeny protostomous 1830s, J. Thompson and C. Brumeister identified the 14. With trochophore larva, and a veliger larva in two larval stages of barnacles and revealed them to be crus- major groups taceans, and in 1866 Alexander Kowalevsky removed the tunicates from Mollusca. Separation of the brachio- pods from the molluscs was long and controversial and not resolved until near the end of the nineteenth possess calcareous sclerites by placing Polyplacophora century. as the sister taxon of the aplacophorans (Caudofoveata The first sclerite-covered wormlike aplacopho- + Solengastres). Aculifera was sometimes also called rans, members of what today we recognize as the class Amphineura, although this latter term has also been Caudofoveata, were discovered in 1841 by the Swedish used by some workers to refer only to chitons. Sclerites naturalist Sven Lovén. He classified them with ho- are spicules, scales, and so on that cover or are embed- lothuroid echinoderms because of their vermiform ded in the epidermis of molluscs and are often calcified. bodies and the presence of calcareous sclerites in the The history of classification of species in the class body walls of both groups. In 1886, another Swede, Gastropoda has been volatile, undergoing constant Tycho Tullberg, described the first representative of change since Cuvier’s time. Most modern malacolo- the other aplacophoran group—the Solenogastres. gists adhere more or less to the basic schemes of Henri Ludwig von Graff (1875) recognized both groups as Milne-Edwards (1848) and J. W. Spengel (1881). The for- molluscs and they were united in the Aplacophora mer, basing his classification on the respiratory organs, in 1876 by Hermann von Ihering. The Aculifera hy- recognized the groups Pulmonata, Opisthobranchia, pothesis of Amélie Scheltema unites molluscs that and Prosobranchia. Spengel based his scheme on the nervous system and divided the gastropods into the 2The name of the phylum derives from the Latin molluscus, mean- Streptoneura and Euthyneura. In subsequent classifica- ing “soft,” in allusion to the similarity of clams and snails to tions, Streptoneura was equivalent to Prosobranchia; the mollusca, a kind of Old World soft nut with a thin but hard Euthyneura included Opisthobranchia and Pulmonata. shell. The vernacular for Mollusca is often spelled mollusks in the United States, whereas in most of the rest of the world it is The bivalves have been called Bivalvia, Pelecypoda, typically spelled molluscs. In biology, a vernacular or diminutive and Lamellibranchiata. More recently, anatomical, name is generally derived from the proper Latin name; thus the ultrastructural, and molecular studies have brought custom of altering the spelling of Mollusca by changing the c to k seems to be an aberration (although it may have its historic roots about considerable changes to molluscan classification, in the German language, which does not have the free-standing c; as outlined below. Many taxa have multiple names and e.g., Molluskenkunde). We prefer the more widely used spelling the more commonly encountered ones are noted below. “molluscs,” which seems to be the proper vernacularization and is in line with other accepted terms, such as molluscan, molluscoid, Molluscan classification at the generic and species molluscivore, etc. levels is also troublesome. Many species of gastropods B&B_3E Chapter 13.indd 454 12/18/15 3:01 PM 08/28/2016 - RS0000000000000000000000125283 - Invertebrates PHYLUM MOLLUSCA 455 and bivalves are also burdened with numerous names COHORT HYPSOGASTROPODA Higher caeno- (synonyms) that have been proposed for the same spe- gastropods cies. This tangle is partly the result of a long history of SUPERORDER LITTORINIMORPHA Periwinkles, amateur shell collecting beginning with the natural his- cowries, triton shells, etc. tory cabinets of seventeenth century Europe, which re- SUPERORDER NEOGASTROPODA Whelks, quired documentation and promoted multiple taxono- volutes, rock shells etc. mies and names based only on shell characters. Today, species are recognized based on a combination of