桃花(hua)水(shui)(shui)(shui)母的(de)生(sheng)(sheng)活史為無(wu)性(xing)生(sheng)(sheng)殖與有性(xing)生(sheng)(sheng)殖世代交替而(er)成,螅(xi)狀(zhuang)體(ti)(ti)(水(shui)(shui)(shui)母的(de)“童年(nian)”)對(dui)環(huan)境(jing)(jing)(jing)要求極低,而(er)一旦分離出水(shui)(shui)(shui)母則對(dui)環(huan)境(jing)(jing)(jing)和水(shui)(shui)(shui)質要求很高(gao)。環(huan)境(jing)(jing)(jing)適應時,螅(xi)狀(zhuang)體(ti)(ti)便自然(ran)分離出水(shui)(shui)(shui)母;環(huan)境(jing)(jing)(jing)不利時,螅(xi)狀(zhuang)體(ti)(ti)便長期吸附(fu)于(yu)水(shui)(shui)(shui)下(xia)或(huo)巖石縫中世代生(sheng)(sheng)存下(xia)去。桃花(hua)水(shui)(shui)(shui)母產于(yu)淡水(shui)(shui)(shui)中,居住人(ren)工湖,池塘、湖泊和河(he)流。對(dui)水(shui)(shui)(shui)環(huan)境(jing)(jing)(jing)的(de)要求極高(gao),適宜其生(sheng)(sheng)存的(de)水(shui)(shui)(shui)域必須無(wu)毒(du)無(wu)害、潔凈,且多呈酸(suan)性(xing)。測出PH值(zhi)為6.4。這一微(wei)酸(suan)水(shui)(shui)(shui)環(huan)境(jing)(jing)(jing)最適宜水(shui)(shui)(shui)母生(sheng)(sheng)長。
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中國四川(成都、都江堰市(shi)、攀枝花、瀘州(zhou)市(shi))、浙江、湖北、江西(xi)南(nan)昌、湖南(nan)長沙、廈門、河南(nan)信陽、東(dong)北松花江流域、臺灣、香港等。
桃花(hua)水(shui)(shui)(shui)母(mu)直徑約(yue)15-25毫(hao)(hao)米(mi)(mi),幾乎是(shi)透明的(de),有白(bai)色或(huo)綠(lv)色色調(diao),觸角(jiao)約(yue)20毫(hao)(hao)米(mi)(mi)長。呈現出傘狀,有螺紋串狀的(de)觸手(shou)圍繞其(qi)圓邊(bian)(環(huan)(huan)形緣)。圓形管的(de)直徑從5至25毫(hao)(hao)米(mi)(mi)。是(shi)僅有的(de)一種淡水(shui)(shui)(shui)生(sheng)活(huo)的(de)小型(xing)(xing)(xing)水(shui)(shui)(shui)母(mu),生(sheng)活(huo)史具有單(dan)體的(de)小型(xing)(xing)(xing)的(de)水(shui)(shui)(shui)螅型(xing)(xing)(xing)體和自由游泳的(de)水(shui)(shui)(shui)母(mu)型(xing)(xing)(xing)體,但以(yi)水(shui)(shui)(shui)母(mu)型(xing)(xing)(xing)為主。水(shui)(shui)(shui)螅型(xing)(xing)(xing)體無(wu)圍鞘,甚至無(wu)觸手(shou),單(dan)體常僅數(shu)毫(hao)(hao)米(mi)(mi)。水(shui)(shui)(shui)母(mu)體也為小型(xing)(xing)(xing),都具緣膜,這是(shi)水(shui)(shui)(shui)螅水(shui)(shui)(shui)母(mu)的(de)特(te)征,具很多觸手(shou)。水(shui)(shui)(shui)螅型(xing)(xing)(xing)結(jie)構簡單(dan),沒(mei)有口(kou)道,胃循環(huan)(huan)腔中也沒(mei)有隔膜。水(shui)(shui)(shui)母(mu)型(xing)(xing)(xing)絕大多數(shu)具有緣膜,胃腔中沒(mei)有刺細(xi)胞。水(shui)(shui)(shui)螅型(xing)(xing)(xing)及水(shui)(shui)(shui)母(mu)型(xing)(xing)(xing)的(de)中膠層(ceng)(ceng)中均(jun)無(wu)細(xi)胞結(jie)構,生(sheng)殖(zhi)細(xi)胞均(jun)來源于(yu)(yu)表(biao)皮層(ceng)(ceng)(外(wai)胚層(ceng)(ceng)),個(ge)別種即使來源于(yu)(yu)內胚層(ceng)(ceng),但最后仍在(zai)外(wai)胚層(ceng)(ceng)中發育成熟。
桃(tao)花水(shui)(shui)母,又稱(cheng)“桃(tao)花魚”、“降落傘魚”,生(sheng)長(chang)于溫帶淡水(shui)(shui)中,其(qi)形(xing)狀如桃(tao)花,并(bing)多在(zai)(zai)桃(tao)花季(ji)節(jie)出現,故得(de)名。其(qi)通體(ti)透(tou)明,像透(tou)明小傘在(zai)(zai)水(shui)(shui)中悠(you)然(ran)漂浮,它(ta)們(men)無(wu)頭無(wu)尾呈(cheng)圓形(xing),晶(jing)瑩透(tou)亮,柔軟如綢,身體(ti)周邊長(chang)滿了(le)觸角(jiao)(jiao),中間長(chang)著五個(ge)呈(cheng)桃(tao)花形(xing)分布的觸角(jiao)(jiao)狀物體(ti)。它(ta)們(men)在(zai)(zai)水(shui)(shui)中一(yi)張一(yi)縮(suo)上下飄蕩,悠(you)然(ran)自得(de)。是一(yi)種(zhong)瀕臨絕跡、古老(lao)而珍稀的腔(qiang)腸動物。
桃花(hua)水母的(de)外(wai)形象一把撐(cheng)開的(de)傘,或一個(ge)倒扣的(de)碗,稱(cheng)為傘體,傘邊緣(yuan)有許多細線(xian)狀的(de)觸手,這是(shi)(shi)它的(de)捕食器官。口朝(chao)向傘下方,位于一條(tiao)管子的(de)末端(duan),具4片唇。食物由此吞(tun)入(ru),消化不了的(de)殘渣,仍由口排出體外(wai),所以桃花(hua)水母嘴和肛(gang)門是(shi)(shi)一個(ge)。傘緣(yuan)有一水平緣(yuan)膜,此為水螅水母的(de)共同(tong)特征。
桃花水母的生殖(zhi)腺(xian)顏色(se)(se)有粉紅色(se)(se)、褐黃色(se)(se)、微(wei)綠色(se)(se)、淡黃色(se)(se)、綠色(se)(se)、白色(se)(se)及淡墨色(se)(se)等(deng)。均隨其種(zhong)(zhong)類(lei)(或種(zhong)(zhong)群(qun))和變種(zhong)(zhong)而(er)不同。
桃花(hua)水(shui)母(mu)(mu)體(ti)的傘體(ti)不停地收(shou)縮與舒(shu)張,將下(xia)傘腔內的水(shui)壓出(chu)體(ti)外,借此朝相反(fan)的方向緩慢游動。當遇到食物時,觸手(shou)上(shang)的刺絲囊即射(she)出(chu)刺絲,刺中被捕獲物,頃刻(ke)將其中毒身亡,以觸手(shou)送入口中,吞(tun)入胃內。桃花(hua)水(shui)母(mu)(mu)多以劍水(shui)蚤、小線蟲、小環(huan)蟲、小蝌蚪(dou)、小魚苗等為食。
桃(tao)花(hua)水(shui)(shui)母常出(chu)(chu)現在(zai)溫暖的(de)水(shui)(shui)域(yu)中(zhong)(zhong),溫度至(zhi)少(shao)為(wei)25℃,但水(shui)(shui)溫不能(neng)高于32℃。在(zai)初(chu)夏(xia)(xia)5月(yue)(yue)(yue)(yue)水(shui)(shui)溫14℃和深(shen)秋11月(yue)(yue)(yue)(yue)水(shui)(shui)溫12℃時都(dou)曾采到,其生存水(shui)(shui)域(yu)的(de)pH為(wei)6.5-8.0,以(yi)每年6-10月(yue)(yue)(yue)(yue)的(de)夏(xia)(xia)秋季節出(chu)(chu)現最多(duo)。這(zhe)是因為(wei)適宜(yi)的(de)水(shui)(shui)溫和充足的(de)食物使其大量繁殖,往往在(zai)7-9月(yue)(yue)(yue)(yue)達到高峰,10月(yue)(yue)(yue)(yue)底或11月(yue)(yue)(yue)(yue)初(chu)后桃(tao)花(hua)水(shui)(shui)母群(qun)體數量迅(xun)速下降(jiang),此后便銷聲匿跡。同一(yi)地點常年(或每年)出(chu)(chu)現的(de)現象十分(fen)(fen)罕見。桃(tao)花(hua)水(shui)(shui)母對日(ri)光(guang)有(you)(you)正向反(fan)應,喜(xi)弱光(guang),有(you)(you)一(yi)定的(de)趨光(guang)性,早、晚(wan)分(fen)(fen)布(bu)在(zai)水(shui)(shui)的(de)上層或表層,中(zhong)(zhong)午光(guang)照強、水(shui)(shui)溫高時則多(duo)分(fen)(fen)布(bu)于水(shui)(shui)的(de)中(zhong)(zhong)下層。故在(zai)晴朗無風的(de)早晨和傍晚(wan)時更易采到。
每到暮春,桃(tao)花(hua)(hua)怒放(fang)時(shi)(shi),它(ta)(ta)們就相繼出(chu)現。桃(tao)花(hua)(hua)水母有粉紅(hong)色的(de)生(sheng)殖腺,透明(ming)的(de)傘頂,在(zai)水中(zhong)浮沉(chen)漂(piao)蕩,有如落水桃(tao)花(hua)(hua),因此,中(zhong)國古代稱(cheng)它(ta)(ta)們為“桃(tao)花(hua)(hua)魚(yu)(yu)”。但又(you)明(ming)確指(zhi)出(chu),桃(tao)花(hua)(hua)魚(yu)(yu)“非(fei)魚(yu)(yu)也”,因其水生(sheng),故(gu)名之曰(yue)魚(yu)(yu);又(you)因其生(sheng)于桃(tao)花(hua)(hua)開(kai)時(shi)(shi),故(gu)名之曰(yue)“桃(tao)花(hua)(hua)魚(yu)(yu)”。
自然(ran)(ran)界的(de)(de)(de)(de)桃(tao)花(hua)水(shui)(shui)母在(zai)低(di)溫(wen)、富(fu)營(ying)(ying)養(yang)及饑餓的(de)(de)(de)(de)不利環境(jing)條(tiao)件(jian)下,螅狀體(ti)可收縮,分(fen)泌(mi)一層堅硬的(de)(de)(de)(de)保護膜包被身體(ti),成為(wei)休眠體(ti)(podocyst)。有(you)(you)學者(zhe)認(ren)為(wei),休眠體(ti)能(neng)通過水(shui)(shui)生(sheng)(sheng)(sheng)植(zhi)物(wu)、動物(wu)(如鳥)或(huo)人(ren)為(wei)的(de)(de)(de)(de)因素(su),而使(shi)其傳播。環境(jing)條(tiao)件(jian)改善(shan)時,休眠體(ti)重新發育為(wei)螅狀體(ti),其生(sheng)(sheng)(sheng)活史得以(yi)繼續。所以(yi)桃(tao)花(hua)水(shui)(shui)母也能(neng)在(zai)房頂(ding)水(shui)(shui)箱中被發現(xian)。也有(you)(you)研究者(zhe)認(ren)為(wei),桃(tao)花(hua)水(shui)(shui)母喜(xi)生(sheng)(sheng)(sheng)活在(zai)富(fu)營(ying)(ying)養(yang)的(de)(de)(de)(de)池塘(tang),且勝過寡營(ying)(ying)養(yang)的(de)(de)(de)(de)大湖泊(bo),寡營(ying)(ying)養(yang)的(de)(de)(de)(de)水(shui)(shui)域中缺少浮游(you)(you)植(zhi)物(wu),當然(ran)(ran)也影響小型浮游(you)(you)動物(wu)的(de)(de)(de)(de)繁殖,桃(tao)花(hua)水(shui)(shui)母就(jiu)沒有(you)(you)了(le)生(sheng)(sheng)(sheng)存的(de)(de)(de)(de)餌料(liao)條(tiao)件(jian),只要不是長(chang)期的(de)(de)(de)(de)嚴重的(de)(de)(de)(de)富(fu)營(ying)(ying)養(yang)化的(de)(de)(de)(de)水(shui)(shui)域,還是有(you)(you)可能(neng)出現(xian)桃(tao)花(hua)水(shui)(shui)母的(de)(de)(de)(de)。
桃(tao)花水(shui)母(mu)在何種(zhong)水(shui)體中出現有(you)兩種(zhong)原因決定:其(qi)一(yi),它(ta)對水(shui)質要求(qiu)極高,周(zhou)邊(bian)環境的細微變(bian)化都會導致其(qi)消(xiao)失(shi);其(qi)二,桃(tao)花水(shui)母(mu)無性生殖(zhi)世代(dai)的水(shui)螅型不發(fa)達,螅狀體大(da)小(xiao)僅有(you)0.5-1.0mm,附著(zhu)在水(shui)下腐敗植物、巖(yan)石和(he)(he)樹樁(zhuang)等物體上,體表有(you)粘液,常(chang)粘附著(zhu)一(yi)些碎屑,將身(shen)體部(bu)分地遮蓋,一(yi)般極難被發(fa)現和(he)(he)認(ren)識,其(qi)萌發(fa)成水(shui)母(mu)體的機制(zhi)尚(shang)不清(qing)楚(chu)。
桃花水(shui)母(mu)(mu)屬水(shui)母(mu)(mu)型(xing)生殖體(ti)。個體(ti)呈退化(hua)的(de)(de)水(shui)母(mu)(mu)型(xing)結構,無(wu)口、無(wu)觸手及感(gan)官,具有大量的(de)(de)生殖細胞(bao)(bao),雌(ci)雄(xiong)異體(ti),但群(qun)體(ti)可(ke)以是(shi)雌(ci)雄(xiong)同(tong)體(ti),生殖細胞(bao)(bao)在釋放之后個體(ti)死去。刺胞(bao)(bao)動(dong)物(wu)中多態現象(xiang)的(de)(de)生物(wu)學意義可(ke)以認為(wei)是(shi)群(qun)體(ti)中個體(ti)之間的(de)(de)勞動(dong)分工,因為(wei)腔(qiang)腸動(dong)物(wu)尚(shang)未出現器官系統(tong)來擔任不(bu)同(tong)的(de)(de)生理機能(neng),而是(shi)通過(guo)群(qun)體(ti)中個體(ti)的(de)(de)形態分化(hua)來擔任不(bu)同(tong)的(de)(de)生理機能(neng),這種(zhong)分化(hua)是(shi)一種(zhong)很原始(shi)的(de)(de)形式,這與腔(qiang)腸動(dong)物(wu)的(de)(de)生活史(shi)及發展(zhan)水(shui)平是(shi)密(mi)切相關的(de)(de)。
身體(ti)呈水(shui)(shui)(shui)螅(xi)(xi)(xi)型(xing)(xing)(xing),生(sheng)(sheng)活史(shi)很(hen)簡單(dan),即由水(shui)(shui)(shui)螅(xi)(xi)(xi)型(xing)(xing)(xing)體(ti)產生(sheng)(sheng)精(jing)(jing)、卵(luan)(luan),并形(xing)成受(shou)精(jing)(jing)卵(luan)(luan)→浮浪幼蟲→水(shui)(shui)(shui)螅(xi)(xi)(xi)型(xing)(xing)(xing)單(dan)體(ti)。生(sheng)(sheng)活史(shi)中沒有(you)水(shui)(shui)(shui)母型(xing)(xing)(xing),有(you)的(de)種甚至(zhi)沒有(you)浮浪幼蟲期。在二態及多(duo)態生(sheng)(sheng)活的(de)種類(lei),生(sheng)(sheng)活史(shi)中有(you)水(shui)(shui)(shui)螅(xi)(xi)(xi)型(xing)(xing)(xing)及水(shui)(shui)(shui)母型(xing)(xing)(xing)階段(duan),其(qi)(qi)水(shui)(shui)(shui)螅(xi)(xi)(xi)型(xing)(xing)(xing)行(xing)無性(xing)(xing)生(sheng)(sheng)殖(zhi),水(shui)(shui)(shui)母型(xing)(xing)(xing)或水(shui)(shui)(shui)母芽(ya)行(xing)有(you)性(xing)(xing)生(sheng)(sheng)殖(zhi),其(qi)(qi)受(shou)精(jing)(jing)卵(luan)(luan)仍發育成水(shui)(shui)(shui)螅(xi)(xi)(xi)型(xing)(xing)(xing)體(ti),其(qi)(qi)生(sheng)(sheng)活史(shi)成為水(shui)(shui)(shui)螅(xi)(xi)(xi)型(xing)(xing)(xing)體(ti)→水(shui)(shui)(shui)母型(xing)(xing)(xing)體(ti)或水(shui)(shui)(shui)母芽(ya)→受(shou)精(jing)(jing)卵(luan)(luan)→浮浪幼蟲→水(shui)(shui)(shui)螅(xi)(xi)(xi)型(xing)(xing)(xing)體(ti)。結果(guo)形(xing)成無性(xing)(xing)世代與有(you)性(xing)(xing)世代的(de)交替現象。
按照前一(yi)種觀(guan)點(dian)(dian),原始的(de)(de)(de)腔(qiang)腸動物(wu)應是(shi)(shi)水(shui)(shui)(shui)螅型(xing)(xing)(xing)(xing)(xing),它(ta)通過有性生(sheng)殖,次(ci)生(sheng)性的(de)(de)(de)產生(sheng)水(shui)(shui)(shui)母(mu)(mu)(mu)(mu)(mu)型(xing)(xing)(xing)(xing)(xing)體(ti),因此形成(cheng)(cheng)了二態或(huo)多態。按照后一(yi)種觀(guan)點(dian)(dian)認為(wei),祖先腔(qiang)腸動物(wu)是(shi)(shi)柔軟的(de)(de)(de)水(shui)(shui)(shui)母(mu)(mu)(mu)(mu)(mu)型(xing)(xing)(xing)(xing)(xing),而水(shui)(shui)(shui)螅型(xing)(xing)(xing)(xing)(xing)是(shi)(shi)幼蟲(chong)(chong)(chong)(chong)期(qi)的(de)(de)(de)持續。這(zhe)后一(yi)種觀(guan)點(dian)(dian)在水(shui)(shui)(shui)螅綱硬水(shui)(shui)(shui)母(mu)(mu)(mu)(mu)(mu)目中得到了證實。因為(wei)硬水(shui)(shui)(shui)母(mu)(mu)(mu)(mu)(mu)類(lei)的(de)(de)(de)受精卵在發(fa)育(yu)中通過浮(fu)(fu)浪幼蟲(chong)(chong)(chong)(chong)及(ji)輻(fu)射(she)(she)幼蟲(chong)(chong)(chong)(chong)(actinula)直接(jie)發(fa)育(yu)成(cheng)(cheng)水(shui)(shui)(shui)母(mu)(mu)(mu)(mu)(mu)型(xing)(xing)(xing)(xing)(xing),其生(sheng)活史是(shi)(shi)水(shui)(shui)(shui)母(mu)(mu)(mu)(mu)(mu)型(xing)(xing)(xing)(xing)(xing)體(ti)→受精卵→浮(fu)(fu)浪幼蟲(chong)(chong)(chong)(chong)→輻(fu)射(she)(she)幼蟲(chong)(chong)(chong)(chong)→水(shui)(shui)(shui)母(mu)(mu)(mu)(mu)(mu)體(ti)。一(yi)種小型(xing)(xing)(xing)(xing)(xing)的(de)(de)(de)水(shui)(shui)(shui)螅Microhydra的(de)(de)(de)個體(ti)發(fa)育(yu)正(zheng)是(shi)(shi)如(ru)此,這(zhe)說明輻(fu)射(she)(she)幼蟲(chong)(chong)(chong)(chong)首先發(fa)展了固著習性,隨(sui)著延(yan)伸固著期(qi),停(ting)止發(fa)育(yu)成(cheng)(cheng)水(shui)(shui)(shui)母(mu)(mu)(mu)(mu)(mu)型(xing)(xing)(xing)(xing)(xing),而最終發(fa)育(yu)成(cheng)(cheng)水(shui)(shui)(shui)螅型(xing)(xing)(xing)(xing)(xing)群(qun)體(ti),所以(yi)生(sheng)活史的(de)(de)(de)改變形成(cheng)(cheng)了多態。這(zhe)種現象與管(guan)水(shui)(shui)(shui)母(mu)(mu)(mu)(mu)(mu)類(lei)的(de)(de)(de)發(fa)育(yu)也(ye)(ye)相符合,它(ta)的(de)(de)(de)浮(fu)(fu)浪幼蟲(chong)(chong)(chong)(chong)也(ye)(ye)是(shi)(shi)首先形成(cheng)(cheng)水(shui)(shui)(shui)母(mu)(mu)(mu)(mu)(mu)型(xing)(xing)(xing)(xing)(xing)體(ti),即(ji)浮(fu)(fu)囊體(ti)、游(you)泳體(ti)。因此世代交替(ti)的(de)(de)(de)觀(guan)點(dian)(dian)應該讓位于水(shui)(shui)(shui)母(mu)(mu)(mu)(mu)(mu)體(ti)是(shi)(shi)成(cheng)(cheng)熟的(de)(de)(de)刺(ci)胞動物(wu),而水(shui)(shui)(shui)螅體(ti)是(shi)(shi)幼蟲(chong)(chong)(chong)(chong)期(qi)的(de)(de)(de)持續階段(duan)的(de)(de)(de)觀(guan)點(dian)(dian)。當然這(zhe)個論點(dian)(dian)是(shi)(shi)建筑(zhu)在硬水(shui)(shui)(shui)母(mu)(mu)(mu)(mu)(mu)目是(shi)(shi)最原始的(de)(de)(de)刺(ci)胞動物(wu)的(de)(de)(de)基礎(chu)之上的(de)(de)(de)。
桃花水(shui)(shui)(shui)(shui)(shui)母(mu)突然出(chu)現(xian),經數日(ri)或(huo)十數日(ri)后又悄悄地消失,這要從它們(men)的(de)(de)(de)(de)生(sheng)殖(zhi)規律來(lai)解答(da)。中(zhong)國在早春(chun)看到的(de)(de)(de)(de)桃花水(shui)(shui)(shui)(shui)(shui)母(mu),為成熟(shu)(shu)的(de)(de)(de)(de)水(shui)(shui)(shui)(shui)(shui)母(mu)體(ti)(ti),有(you)雌(ci)(ci)雄(xiong)之分。觸(chu)手細長,伸向上(shang)(shang)方的(de)(de)(de)(de)均為雌(ci)(ci)性,觸(chu)手短粗,垂向下方的(de)(de)(de)(de)為雄(xiong)性。雌(ci)(ci)雄(xiong)將卵子和(he)精子排到水(shui)(shui)(shui)(shui)(shui)中(zhong),卵受精發育成一個微小的(de)(de)(de)(de)滿被纖毛的(de)(de)(de)(de)浮游幼蟲,后幼蟲一端接觸(chu)石土(tu)央等外(wai)物(wu),固著(zhu),發育成一個極(ji)小的(de)(de)(de)(de)樹枝狀的(de)(de)(de)(de)水(shui)(shui)(shui)(shui)(shui)螅體(ti)(ti)。水(shui)(shui)(shui)(shui)(shui)螅體(ti)(ti)可度過酷熱的(de)(de)(de)(de)夏季(ji)和(he)嚴寒的(de)(de)(de)(de)冬(dong)季(ji),待來(lai)年春(chun)天,以出(chu)芽生(sheng)殖(zhi)產(chan)(chan)生(sheng)水(shui)(shui)(shui)(shui)(shui)母(mu)體(ti)(ti)。水(shui)(shui)(shui)(shui)(shui)母(mu)體(ti)(ti)成熟(shu)(shu)后,再進(jin)行(xing)有(you)性生(sheng)殖(zhi)。水(shui)(shui)(shui)(shui)(shui)螅體(ti)(ti)長0.3毫米(mi),水(shui)(shui)(shui)(shui)(shui)母(mu)體(ti)(ti)進(jin)行(xing)有(you)性生(sheng)殖(zhi)產(chan)(chan)生(sheng)水(shui)(shui)(shui)(shui)(shui)螅體(ti)(ti),水(shui)(shui)(shui)(shui)(shui)螅體(ti)(ti)進(jin)行(xing)無性生(sheng)殖(zhi)產(chan)(chan)生(sheng)水(shui)(shui)(shui)(shui)(shui)母(mu)體(ti)(ti),這在動物(wu)學(xue)上(shang)(shang)稱為世代交替。