This article was downloaded by: 10.3.98.104 On: 25 Sep 2021 Access details: subscription number Publisher: CRC Press Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: 5 Howick Place, London SW1P 1WG, UK Biology and Evolution of the Mollusca Winston F. Ponder, David R. Lindberg, Juliet M. Ponder Shell, Body, and Muscles Publication details https://www.routledgehandbooks.com/doi/10.1201/9781351115667-3 Winston F. Ponder, David R. Lindberg, Juliet M. Ponder Published online on: 01 Nov 2019 How to cite :- Winston F. Ponder, David R. Lindberg, Juliet M. Ponder. 01 Nov 2019, Shell, Body, and Muscles from: Biology and Evolution of the Mollusca CRC Press Accessed on: 25 Sep 2021 https://www.routledgehandbooks.com/doi/10.1201/9781351115667-3 PLEASE SCROLL DOWN FOR DOCUMENT Full terms and conditions of use: https://www.routledgehandbooks.com/legal-notices/terms This Document PDF may be used for research, teaching and private study purposes. Any substantial or systematic reproductions, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The publisher shall not be liable for an loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. 3 Shell, Body, and Muscles In this chapter, we provide an overview of the external body on the taxon. In swimming Nautilus and crawling octopods, of molluscs, comprising the mantle, the shell and its forma- the ventral axis is rotated about 60°, while in cuttlefish and tion and growth, the epidermis and associated structures, the squid, the body axis is rotated a full 90° in its life orientation foot and operculum (of gastropods), mucoid secretions, loco- (Figure 3.1). motion, and general information on cartilage and muscles. Early molluscs were probably1 bilaterally symmetrical, Some external structures, such as the suckers found in many although the adult orientation of the anterior-posterior axis of coleoid cephalopods, are also important external features, but the early mollusc is less certain and largely depends on the as they are confined to a single group, we deal with them in putative sister taxon and outgroups. If the traditional mollusc- the appropriate taxon chapter. annelid relationship is favored, the adult orientation of the early molluscan body extended along an anterior-posterior axis with a low dorsoventral axis, as in chitons, aplacopho- 3.1 BODY SYMMETRY AND AXES rans, and monoplacophorans (Figure 3.1). In most of these Molluscan body axes are complicated, and there can be up animals, the ventral and dorsal orientation of the body is to three distinct body orientations which transform dur- maintained throughout life. ing ontogeny. As described in more detail in Chapter 8, in Besides anatomical and ecological changes in body axes, most bilaterally symmetrical animals the embryonic blastula molluscan evolution has been characterised by organ asym- has an animal-vegetal polarity, and during gastrulation, the metry and displacement. The rotation of the viscera on the blastopore forms at the vegetal pole (Biggelaar et al. 2002). head-foot in gastropod larvae during torsion is a famous In deuterostomes, the blastopore becomes the anus, thus example of twisting. Numerous additional changes in body establishing the vegetal pole as the posterior (P) of the embryo axes occur in gastropods (Figure 3.2), including detorsion, and the animal pole as the anterior (A). In protostomes, the which results in the movement of the mantle cavity posteriorly initially posterior vegetal blastopore is rotated from its ini- along the right side of the body. Some ‘pulmonate’ gastropods tial position to an anterior-ventral position where it typically have a posterior opening to the modified mantle cavity (as a forms the mouth opening (Biggelaar et al. 2002). In molluscs, lung), and this cavity is lost in some other heterobranchs (see this reorientation of the embryonic A-P axis during the forma- Chapters 4 and 20). tion of the trochophore larvae is driven by cell proliferation Bivalves have also undergone considerable modification. and migration on the embryonic dorsal surface which dis- Their shell is divided into two valves, accompanied with con- places the blastopore ventrally and then anteriorly (Biggelaar siderable lateral compression, and the mantle cavity typically et al. 2002; Wanninger & Wollesen 2015). Thus, by the mol- surrounds all or most of the body. Changes in orientation luscan trochophore stage, the original embryonic A-P axis is largely result from byssal, as in mussels, or cement attach- no longer linear, and the originally posterior blastopore has ment, as in oysters (see Figure 3.3 and Chapter 15). shifted approximately 90° to the ventral surface. In shelled In chitons, aplacophorans, and monoplacophorans the taxa, further cell proliferation and migration resulting from viscera (visceral mass) is distributed along the body, but in the formation of the dorsal shell gland displaces the second- bivalves, it is compacted laterally and dorsally, allowing ary anal opening ventrally and anteriorly. This produces the space for an expansive mantle cavity between the enclosing characteristic U-shaped gut indicative of ano-pedal flexure shell valves. In scaphopods and cephalopods, it is elongated and obscures the original embryonic body axes. A third reori- dorsally. In Nautilus (and many fossil cephalopods such as entation of the body axes may occur after metamorphosis the ammonites) this elongated tube has been coiled to enable when the animal assumes its adult life orientation. Life (or more efficient packing of the viscera. In gastropods, the dor- ‘ecological’) orientation often differs from the anatomical sally extended body is also coiled, but usually asymmetrically axes in molluscs (Figure 3.1). Some caudofoveates burrow in into a helical coil. the sediment with the posterior end dorsal, while scaphopods burrow obliquely to near vertically in the sediment with the head and foot positioned ventrally. In living cephalopods, the ventral axis is typically rotated from 45° to 90° depending 1 Not necessarily true if Brachiozoa are the sister taxon. Downloaded By: 10.3.98.104 At: 22:14 25 Sep 2021; For: 9781351115667, chapter3, 10.1201/9781351115667-3 55 56 Biology and Evolution of the Mollusca Polyplacophora Monoplacophora Aplacophora Scaphopoda Gastropoda Bivalvia Cephalopoda dorso-ventral axis of body gut antero-posterior axis of body, including viscera FIGURE 3.1 Anatomical axes in Mollusca. All animals in life orientation. Some aplacophorans are buried in an inclined or near vertical orientation, others lie horizontally. Original. Patellid (Patellogastropod) Vermetid Early planispiral (Caenogastropod) gastropod Trochid (Vetigastropod) Olivid (Caenogastropod) Heteropod Cephalaspidian (Heterobranch) Doridid (Heterobranch) (Caenogastropod) --- dorso-ventral axis --- axis of volution --- life-orientation axis FIGURE 3.2 Axis of symmetry in Gastropoda showing body and shell axes relative to life orientation. Redrawn and modified from Morton, J.E. and Yonge, C.M., Classification and structure of the Mollusca, pp. 1–58, in K.M. Wilbur and Yonge, C.M., Physiology of Mollusca, Vol. 1, Academic Press, New York, 1964. Downloaded By: 10.3.98.104 At: 22:14 25 Sep 2021; For: 9781351115667, chapter3, 10.1201/9781351115667-3 Shell, Body, and Muscles 57 INFAUNAL EPIFAUNAL Mytilid (Pteriomorphia) Pteriid (Pteriomorphia) Venerid (Heterodonta) Solenid (Heterodonta) Pectinid (Pteriomorphia) --- dorsoventral axis --- anteroposterior axis Ostreid (Pteriomorphia) FIGURE 3.3 Axes of symmetry in bivalves. Original. 3.2 SHELLS AND SPICULES single shell (univalved), and the shell is secondarily lost in some members of the latter two groups. Molluscan shells have attracted the interest of a wide range of While most gastropods are univalved, a few secrete scientists from many disparate disciplines as well as natural- an accessory shell valve after the primary shell is formed ists, artists and collectors, and the public. (e.g., hipponicids and bivalved sacoglossans). Like the oper- A calcium carbonate shell or spicules is the most obvious culum, these second shells are not true valves because they molluscan marker. Whether these shells were formed from are not secreted by the shell gland. fused spicules or not is still debated (see Chapter 13). In living In aplacophorans, the body is covered by spicules embed- molluscs, only aplacophorans exclusively have spicules, while ded in a cuticle covering the epidermis. A cuticle also cov- chitons have eight shell plates surrounded by a girdle covered ers the dorsal surface of chitons and the girdle ornamentation with spicules, scales, or hairs. (spines, scales) is embedded in it, and in some taxa it is devel- The earliest putative mollusc shells (Rostroconchia) in the oped into hairs. Calcareous dermal spicules are also present fossil record are from the early Cambrian (535–530 mya). in some heterobranch slugs (see Chapter 20). Disarticulated plates, cap-shaped, and simple curved or The molluscan shell is a calcareous structure that covers coiled ‘shells’ are also present during this period and have some to all of the upper surface of the animal. It is generally been interpreted as being shell