舌形貝俗名(ming)海(hai)(hai)豆芽,是世(shi)界上(shang)已發現(xian)生(sheng)(sheng)物(wu)中(zhong)生(sheng)(sheng)存歷史最長的(de)一(yi)個屬(shu),是著名(ming)的(de)“活(huo)化石”,生(sheng)(sheng)活(huo)在溫帶(dai)和熱帶(dai)海(hai)(hai)域(yu)。殼(ke)(ke)為幾丁磷灰質,兩殼(ke)(ke)大小近(jin)等(deng),輪廓舌形或(huo)長卵(luan)形,兩殼(ke)(ke)瓣皆稍(shao)稍(shao)隆(long)起,后(hou)緣尖縮,前緣平直。腹殼(ke)(ke)有一(yi)小的(de)假鉸(jiao)合面。殼(ke)(ke)面平滑,或(huo)具同心(xin)紋。有一(yi)長柄。常見于(yu)寒武系(xi)以來的(de)海(hai)(hai)相(xiang)地(di)層,現(xian)生(sheng)(sheng)種至今仍未絕滅,分布于(yu)世(shi)界各地(di)。
舌形(xing)貝(bei)呈(cheng)(cheng)殼(ke)(ke)舌形(xing)或長(chang)卵(luan)形(xing),后緣(yuan)(yuan)尖縮(suo),前(qian)緣(yuan)(yuan)平直(zhi)。兩殼(ke)(ke)凸度相似(si),大小近(jin)等(deng),但腹殼(ke)(ke)略長(chang)。殼(ke)(ke)壁脆薄,殼(ke)(ke)多糖和磷灰質交互成(cheng)層(ceng)。殼(ke)(ke)面具油脂光澤,飾(shi)以同(tong)心紋(wen)(wen)。肉(rou)莖(jing)特長(chang),自(zi)兩殼(ke)(ke)間伸出,深埋(mai)于潛穴中,并在(zai)腹殼(ke)(ke)假鉸合面上留下一(yi)個三角(jiao)形(xing)的凹溝,稱為肉(rou)莖(jing)溝(pedicle groove)。外套膜(mo)邊緣(yuan)(yuan)具剛毛,促使水由前(qian)方(fang)兩側進入腕腔,再由前(qian)方(fang)中央排出。小舌形(xing)貝(bei)兩殼(ke)(ke)大小相等(deng),長(chang)卵(luan)形(xing)至亞三角(jiao)形(xing),前(qian)緣(yuan)(yuan)圓。腹殼(ke)(ke)后緣(yuan)(yuan)比較尖銳,有(you)清晰的假鉸合面和莖(jing)溝。背殼(ke)(ke)稍短(duan)。殼(ke)(ke)面具同(tong)心紋(wen)(wen),有(you)時呈(cheng)(cheng)斷續的層(ceng)狀,或具放(fang)射紋(wen)(wen)。
舌形(xing)(xing)貝(bei)類(lei)(lei)是(shi)無鉸小腕足類(lei)(lei),殼(ke)由殼(ke)多(duo)糖組(zu)成(cheng)(cheng)。現(xian)生屬(海(hai)豆芽屬〔lingula〕)見(jian)于(yu)(yu)正常的(de)海(hai)洋環(huan)境(jing),但在(zai)不(bu)(bu)適于(yu)(yu)大(da)多(duo)數生物生活的(de)多(duo)泥、缺氧的(de)半咸(xian)水中更為(wei)常見(jian)。小舌形(xing)(xing)貝(bei)屬(lingulella)是(shi)寒武(wu)(wu)系的(de)化石,外(wai)(wai)形(xing)(xing)和(he)構(gou)造上都與現(xian)代海(hai)豆芽屬類(lei)(lei)似(si)。鱗(lin)舌形(xing)(xing)貝(bei)屬(Lingulepis,大(da)致(zhi)限於晚寒武(wu)(wu)世)外(wai)(wai)形(xing)(xing)不(bu)(bu)同于(yu)(yu)其他舌形(xing)(xing)貝(bei)類(lei)(lei),形(xing)(xing)態更像淚滴。舌形(xing)(xing)貝(bei)類(lei)(lei)是(shi)提(ti)供環(huan)境(jing)信(xin)息的(de)有用化石;對(dui)于(yu)(yu)地層(ceng)對(dui)比作用不(bu)(bu)大(da);是(shi)寒武(wu)(wu)紀腕足動物群的(de)重要成(cheng)(cheng)員。(舌形(xing)(xing)貝(bei)的(de)歷史距今(jin)已(yi)有4.5億(yi)年左右,它們起(qi)源于(yu)(yu)寒武(wu)(wu)紀。)