海洋无脊椎动物 4.15 Xenoturbellids( fig 4.70a ) 1 Hermaphrodite, flatwormilike animals that occur on soft marine sediments in northwestern Europe. 2 许多特点类似扁虫,但是有些细微的构造和後口动物相似 3 曾被认为是paedomophic deuterstome larva,最近根据胚胎发生学认为 他的关系和bivalve molluscs较接近,且是经过dedifferentiation的 metamorphosed trochophore larva. Buddenbrockids( fig 4.70b ) 1 没有肠和神经系统,有点类似旋转虫( rhombozoan )或直游虫( orthonectidans ) 2 possesses longitudinal tracts of muscle fibers and outer layer of cells are no ciliated 3 运动方式像线虫 Lobatocerebrids 1 hermaphrodite interstitial worm 2 occur on soft marine sediments in North Atlantic and Red Sea 3 表面上非常像free living扁虫,但是在体壁的构造、肠和雄性的生殖 系统都较类似环节动物 4 Two contrasting views: (i) It is a paedomorphic larva of some larger, possibly polychaete annelid, worm. (ii) It is an early acoelomate offshoot of the line that ultimately gave rise to the schizocoelomate worms. Orthonectidans直游虫 1 Bilaterally symmetrical,in the sexual phase at least. 2 Parasite unattached within the tissue of marine turbellarian, annelids, nemertines, mollusks and echinoderms 3 Helical symmetry 4 Two phase,sexual and asexual. The sexes is separate in sexual phase and dimorphic. 5 身体结构是一层轮状排列的体细胞围绕住一团配子,体细胞下层有环肌和纵肌 6 精子由雄性个体释出进入雌体,在慈体内授精後会形成ciliated larva（实际上是一层具有纤毛的细胞包围着一层germinal cells）在雌体内发育 7 The germinal cells are considered to be mesodermal in origin. 8 当larva释出侵入寄主後,会除去具有纤毛的细胞而留下germinal cells 继续增殖形成multinucleate plasmodium,这些plasmodium可以形无性分 裂形成另外的plasmodium 9 A single plasmodium producing either those of a single sex or both males and females, and these sexual phases then leave the host and complete the life history. -- 凌云吾志 宇宙翔行 劲风展翼 鹰扬千里 --

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