Raikova E V
Cell Tissue Res. 1980;206(3):487-500. doi: 10.1007/BF00237977.
The larval stage of Polypodium hydriforme is planuliform and parasitic inside the growing oocytes of acipenserid fishes. The larva has inverted germ layers and a special envelope, the trophamnion, surrounding it within the host oocyte. The trophamnion is a giant unicellular provisory structure derived from the second polar body and performing both protective and digestive functions, clearly a result of adaptation to parasitism. The trophamnion displays microvilli on its inner surface, and irregular protrusions anchoring it to the yolk on its outer surface. Its cytoplasm contains long nuclear fragments, ribosomes, mitochondria, microtubules, microfilaments, prominent Golgi bodies, primary lysosomes, and secondary lysosomes with partially digested inclusions. The cells of the larva proper are poorly differentiated. No muscular, glandular, neural, interstitial, or nematocyst-forming cells have been found. The entodermal (outer layer) cells bear flagella and contain rough endoplasmic reticulum; the ectodermal (inner layer) cells lack cilia and contain an apical layer of acid mucopolysaccharid granules. The cells of both layers contain mitochondria, microtubules, and Golgi bodies; their nuclei display large nucleoli with nucleolonema-like structure, decondensed chromatin, and some perichromatin granules. At their apical rims, the ectodermal cells from septate junctions; laterally, the cells of both layers form simple contacts and occasional interdigitations. The lateral surfaces of entodermal cells are strengthened by microtubules.
多盘虫的幼虫阶段呈扁平状，寄生于鲟形目鱼类正在发育的卵母细胞内。幼虫具有反转的胚层，并且在宿主卵母细胞内有一个特殊的包膜，即滋养膜围绕着它。滋养膜是一个巨大的单细胞临时性结构，源自第二极体，兼具保护和消化功能，显然是适应寄生生活的结果。滋养膜内表面有微绒毛，外表面有不规则突起将其固定在卵黄上。其细胞质包含长核片段、核糖体、线粒体、微管、微丝、显著的高尔基体、初级溶酶体和含有部分消化内含物的次级溶酶体。幼虫本身的细胞分化程度很低。未发现肌肉、腺体、神经、间质或刺细胞形成细胞。内胚层（外层）细胞有鞭毛，含有粗面内质网；外胚层（内层）细胞没有纤毛，含有一层顶端酸性粘多糖颗粒。两层细胞都含有线粒体、微管和高尔基体；它们的细胞核有大核仁，具有核仁线样结构、解聚的染色质和一些核周颗粒。在外胚层细胞的顶端边缘形成分隔连接；在侧面，两层细胞形成简单连接和偶尔的指状交叉。内胚层细胞的侧面由微管加强。
