Scheltema Amélie H, Kerth Klaus, Kuzirian Alan M
Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA.
J Morphol. 2003 Aug;257(2):219-45. doi: 10.1002/jmor.10121.
As the original molluscan radula is not known from direct observation, we consider what the form of the original radula may have been from evidence provided by neomenioid Aplacophora (Solenogastres), Gastropoda, Polyplacophora, and the Cambrian fossil Wiwaxia corrugata (Matthews). Conclusions are based on direct observation of radula morphology and its accessory structures (salivary gland ducts, radular sac, anteroventral radular pocket) in 25 species and 16 genera of Aplacophora; radula morphogenesis in Aplacophora; earliest tooth formation in Gastropoda (14 species among Prosobranchia, Opisthobranchia, and Pulmonata); earliest tooth formation in four species of Polyplacophora; and the morphology of the feeding apparatus in W. corrugata. The existence of a true radula membrane and of membranoblasts and odontoblasts in neomenioids indicates that morphogenesis of the aplacophoran radula is homologous to that in other radulate Mollusca. We conclude from p redness of salivary gland ducts, a divided radular sac, and a pair of anteroventral pockets that the plesiomorphic state in neomenioids is bipartite, formed of denticulate bars that are distichous (two teeth per row) on a partially divided or fused radula membrane with the largest denticles lateral, as occurs in the genus Helicoradomenia. The tooth morphology in Helicoradomenia is similar to the feeding apparatus in W. corrugata. We show that distichy also occurs during early development in several species of gastropods and polyplacophorans. Through the rejection of the null hypothesis that the earliest radula was unipartite and had no radula membrane, we conclude that the original molluscan radula was similar to the radula found in Helicoradomena species.
由于无法通过直接观察得知原始软体动物的齿舌形态，我们根据新碟贝形无板纲动物（单板纲）、腹足纲、多板纲以及寒武纪化石皱瘤海百合（马修斯）提供的证据来推测原始齿舌可能的形态。研究结论基于对以下方面的观察：25个物种和16个属的无板纲动物的齿舌形态及其附属结构（唾液腺导管、齿舌囊、前腹侧齿舌袋）；无板纲动物的齿舌形态发生；腹足纲动物（前鳃亚纲、后鳃亚纲和肺螺亚纲中的14个物种）最早的齿形成；4种多板纲动物最早的齿形成；以及皱瘤海百合进食器官的形态。新碟贝形无板纲动物中存在真正的齿舌膜、成膜细胞和造齿细胞，这表明无板纲动物齿舌的形态发生与其他有齿舌的软体动物是同源的。我们从唾液腺导管的特征、分开的齿舌囊以及一对前腹侧袋得出结论，新碟贝形无板纲动物的原始状态是二分的，由齿状条组成，在部分分开或融合的齿舌膜上呈双列排列（每行两颗齿），最大的齿在侧面，如在螺旋新碟贝属中所见。螺旋新碟贝属的齿形态与皱瘤海百合的进食器官相似。我们还表明，双列排列在几种腹足纲动物和多板纲动物的早期发育过程中也会出现。通过否定最早的齿舌是不分叶且没有齿舌膜的零假设，我们得出结论，原始软体动物的齿舌与螺旋新碟贝属物种中的齿舌相似。
