系棘皮动物之水管系中连接筛板和环管的管，一般因壁内含钙而坚硬，故有此名。但海胆等亦有完全不含钙的种类（紫海胆）。海参仅含极微小的小骨片。石管周围为轴器所包。
“我们向客户推销，我们对他们说，‘瞧，你推广销售需要的钱都在这里了；让我们看看销量有多少增长，’”顶石高管约瑟夫·伊古萨解释说。
"We're pushing [our clients], [saying,] 'Look, you have all the money you need to finance sales; let's see some sales growth,'" explains Joseph Ingrassia, managing member of Capstone.
现有的白色二极管同样基于蓝色的光——源于这种颜色的光是用于激发一种由钇铝石榴石组成的磷光层。
Existing white LEDs are also based on blue ones, the light from which is used to excite a phosphor layer made of yttrium aluminium garnet (YAG).
即使当技术具备有改变的潜力时，在第三世界的管控常常由于组织性的缺失而成为绊脚石。
Even when technologies have the potential to make a difference, deployment in the developing world often stumbles for lack of infrastructure.
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