海绵动物门和扁盘动物门

1、第四章 多孔动物门（Porifera）海绵动物门Spongia 附：扁盘动物门(Placozoa) 海绵动物为多细胞动物，身体由皮层和领鞭毛细胞的 胃层组成，有独特的水沟系统。胚胎发育等方面与其它多 细胞动物显著不同。一般认为海绵动物是多细胞动物进化 中的一个侧支。,第一节 海绵动物的形态结构,（一）体型多数不对称,（二）没有器官系统和明确的组织 由两层细胞构成，外层称为皮层，内层称为胃层（又 称领细胞层）。2层细胞之间是中胶层。皮层一般由扁平细 胞构成。扁平细胞之间有一些中央有一孔的孔细胞，孔细 胞可收缩，能调节孔的大小。胃层由领鞭毛细胞组成。领 细胞结构似塑料羽毛球状构造。食物颗粒经领鞭毛

2、细胞吞 噬，在细胞内形成食物泡，进行胞内消化，或将食物转移 到中胶层内的变形细胞进行细胞内消化。不能消化的食物 残渣也由变形细胞经水流排除体外。皮层和胃层之间有非 细胞结构的中胶层，其中有变形细胞和骨针。 骨针形态有种的变异。骨针一般由钙质、硅质或角质 构成。,（三）具有水沟系,第二节 海绵动物的生殖和发育,无性生殖：出芽生 殖、形成芽球（以度过 不良环境） 有性生殖：异体受 精。精子通过领鞭毛 时，领鞭毛细胞脱去领 与鞭毛，成为变形虫 状，然后将精子带入中 胶层与卵细胞受精。胚 胎发育中有逆转现象。 海绵动物没有专门的呼 吸和排泄器官，也没有 神经系统。,第三节 海绵动物门的分类及分类地位,

3、钙质海绵纲（Calcarea） ：骨针钙质。 六放海绵纲（Halichondria） ：骨针质地硅质，骨针大，为三轴六辐 或六辐的倍数。水沟系统复沟型。 寻常海绵纲（Demospongiae） ：骨针质地为角质的海绵丝或硅质， 但不是六放的。水沟系为复沟型。 海绵的结构与机能的原始性，很多与原生动物相似，其体内又具有 与原生动物领鞭毛虫相同的领细胞，因此过去有人认为它是与领鞭毛虫 有关的群体原生动物。但是海绵在个体发育中有胚层存在，而且海绵动 物的细胞不能像原生动物那样无限制地生存下去，因此肯定海绵是属于 多细胞动物。近年来生化研究证明，海绵动物体内具有与其他多细胞动 物大致相同的核酸和氨基酸

4、，更加证明了这一点。但海绵的胚胎发育又 与其他多细胞动物不同，有逆转现象，又有水沟系、发达的领细胞、骨 针等特殊结构，这说明海绵动物发展的道路与其他多细胞动物不同，所 以认为它是很早由原始的群体领鞭毛虫发展来的一个侧支，因而称为侧 生动物。,第四节 海绵动物的经济价值,用于液体吸收物，擦洗机器；用于科学研究（细胞 和胚胎发育）,小结,体制不对称或辐射对称，在水中营固着生活；身体 由2层细胞及其之间的中胶层构成；胚胎发育中有逆转的 现象；具特殊的水沟系统；细胞没有组织分化；通常具 有钙质、硅质或角质的骨骼；没有消化腔，只行细胞内 消化；没有神经系统；海绵动物仍然有保存了领鞭毛细 胞；综上所述海绵

5、动物是一类极为原始的多细胞动物， 是多细胞动物进化中的一个侧枝。,1971年德国学者Grell将丝盘虫(Trichoplax adhaerens Schulze)建立为扁盘动物门。该虫的形状，大小，运动方 式与变形虫相似，但它属多细胞动物，最大个体仅 4mm，无前后之分，也无口等器官，整个虫体覆盖一层 具鞭毛的上皮细胞，背面细胞稍扁平，腹面细胞呈柱 状，背腹上皮之内充满来源于腹面上皮的实质组织 (parenchyma)，内有许多变形细胞。行出芽生殖和有性 生殖。胚胎发育无逆转现象。目前其分类地位未确定， 有的认为是吞噬动物门(Phagocytozoa)，立为一亚界；有 的将其列入侧生动物，但又

6、无逆转现象。,附：扁盘动物门（Placozoa）,Only a single species, Trichoplax adhaerens, is known in this phylum. Like the other Parazoa, it lacks tissues, organs and organ systems, head and tail, and left and right. Soft bodied and inconspicuous, it is the simplest of animals. Trichoplax looks like a very large amoeb

7、a; it is barely visible to the naked eye. One can see under higher magnification that the amoeba is really an animal composed of a few thousand cells. Because most of the surface cells bear undulipodia (cilia), the entire surface of the animal is ciliated. Trichoplax adhaerens, the simplest of all a

8、nimals, found adhering to and crawling on the walls of marine aquaria.,Etymology:- From the Greek Plakos for flat and Zoon for animal, hence Placozoa is a flat animal. Characteristics of Placozoa: 1) Has no symmetry or constant shape. 2) Has no distinct tissues or organs. 3) Has no nervous system. 4

9、) Has no body cavity or digestive cavity. 5) Body is shaped like a flat disc. 6) Body consists of a two layers of single cells. 7) Lives in marine environments. 8) One species only known Trichoplax adhaerens.,First discovered in 1883 in a marine aquarium in Austria they were until recently considere

10、d to be planuloid larvae of some kind. Then in the 1960s it was discovered that they could reproduce sexually and they were then given their own phylum. T. adhaerens is a small flat grayish animal growing to a maximum of 3mm in diametre, its body is made up of a few thousand cells at most. Its body

11、though basically disc shaped changes shape in like an amoeba as it moves around. It has no front or back and moves in any direction, gliding along on its cilliated under surface. It has only two layers of cells and 4 different types of cells making it the simplest of all multicellular organisms. The

12、 two cell layers, its top and bottom surface enclose a fluid filled space containing a network of stellate fibre cells. It has a surprisingly small amount of DNA in its cells, similar to species of bacteria, rather than other metazoans.,T. adhaerens is a marine animal, but it is very seldomly recogn

13、ized or identified and its known distribution reflects the distribution of marine biology research stations rather than its true geographic distribution. It feeds on protozoans which it covers with its body and then secretes digestive enzymes onto, the resulting nutrients are absorbed by the same cells that secretethe enzymes. Reproduction can be asexual by either binary fission or budding and accidental cutting of the body into several parts results in each part becoming a new smaller placozoan. Eggs and sperm are known to be produced but nothing else is