海洋动物 第一段：讲海洋里有个sea butterfly，总要抓一个虾在它背上，
然而抓了这个虾在他背上以后，会影响它的mobility,不易抓住pray,在predator出现的时候跑得慢。那它为什么抓这个虾呢？这引起了科学家的好奇。 第二段：然后科学家做个试验，把这个 sea butterfly的predator和它以及这个虾放在一个池子里，发现这个predator不会攻击这个sea butterfly。发现原来它背上的这个东西会发出一种细菌（还是别个）来抑制其predator的攻击行为。 第三段：然后说他们的这个relationship 不是以及广为人知的parasitism（寄生共生）, commensalism（互利共生）,还有个无关共生（英文记不得了）。parasitism是一方是以另一方的伤害一次收益，commensalism是说双方相互获利，那个无关共生说他们的存在对彼此不造成影响（neutral）然后解释了下这三个的关系。然后说这个鱼和他背上的这个虾的这种关系不属于这三个范畴类，应该是个Antagonistic symbiosis,然后说了下为什么不属于parasitism 和无关共生。这个sea butterfly和 decorator crabs还不一样，crab 随便捡周围有的东西，而sea butterfly 只捡这个虾。因为那个鱼把虾子背在背上，但是没有伤害它，不过由于虾子被背在背上，它就不能feed了。
而研究者发现，这个鱼背的虾都是活的，说明它过段时间就换个虾子。 题目：
1、
主旨题：全文应该是解释了一种phenomenon 2、
第一段H鱼把S背在背上，导致了什么结果?
H鱼的移动速度和找食物变慢，
3、
HS组合不是这三种分类，能推出什么？
一个获利，另一个不受伤害
4、
结构题：最后一段（第三段）的作用是什么？
要特别注意第三段，考了两题 原文 The nature of this relationship is puzzling. It is difficult to discern辨别 how carrying another organism around can be advantageous. We speculated that in fact, amphipods无脊椎动物 must be considerably slowed down. By carefully measuring the swimming speeds of amphipods with and without sea butterflies, we found that amphipods carrying sea butterflies moved only half as quickly as similarly sized solitary amphipods. The situation simply made no sense. In reducing their mobility, the amphipods became more vulnerable易受攻击的 to predators and less adept at capturing prey. Why, then, would amphipods go out of their way to abduct and carry sea butterflies? Our experiments clearly demonstrated that something about the sea butterflies was repelling抑制 the fish, and we suspected that this deterrence was chemical. To find out, we conducted a second set of feeding experiments.
We homogenized the sea butterflies and mixed the homogenate with fish-meal powder to make food pellets. As a control, we also made food pellets containing just the fish-meal powder. We offered both the experimental and the control pellets to fish, which always ate the control pellets and always rejected the pellets containing the homogenate. This provided compelling evidence that compounds might be responsible for the feeding chemical deterrence. 中间删去一段~没什么用 Our experiments demonstrated to us that the sea butterflyC. antarctica synthesizes a deterrent substance that the amphipod H. dilatata exploits for its own protection. This unique association - the abduction of one species by another - is unprecedented in the annals of behavioral and chemical ecology. Some decorator crabs are chemical ecology known to cover their uppercarapace with a variety of objects, including the occasional sponge that might harbor defensive chemistry. But this appears to be a nonselective behavior.Crabs haphazardly decorate themselves with whatever is at hand. The association between the sea butterfly and the amphipodfalls within the definition of symbiosis共生, where two dissimilar species live together in an intimate association. However, none of the relationships defined within the broad context of symbiosis - parasitism, comensalism or mutualism -appear to suitably describe the sea butterfly-amphipod relationship. Parasitism寄生 implies that one species associates with another to the detriment伤害of one of them. Often, the parasite feeds off the tissues or body fluids of its host. Mutualism共生describes a relationship where both species benefit from the association.Commensalism describes organisms that live in benign and neutral中立or良性association with one another. Neither of the latter two interactions appropriately describes what we have observed, and parasitism provides only a very weak analogy to it. We feel that a new term might be needed to describe the "antagonistic symbiosis" that seems to describe most accurately this unique interaction. 寄生:一个伤害另一个，吃对方组织和体液。共生:双方都获利。commensalism共栖:中立，互相不干扰。实际(如果加上下一段的文章):一个获利，另一个间接受害,若没有下面这段,得看选项怎么说了.网上的定义是the association is disadvantageous or destructive to one of the organisms, but ordinarily it is used of cases where the association is advantageous, or often necessary, to one or both, and not harmful to either. In this association the antagonist benefits greatly from the relationship. Remember that the amphipod must sacrifice its mobility and speed. Clearly the defense it acquires offsets these drawbacks. The sea butterfly, on the other hand, is at the mercy of the amphipod. While it is being carried around, it cannot feed to sustain its energy | 
