propygidium

例句：

1. 1. The propygidium is a unique and fascinating structure found in certain species of fish.
   这个结构是某些鱼类特有的，非常独特且引人入胜。
2. 2. The propygidium plays a crucial role in the functioning of the fish's gills.
   这个结构在鱼的鳃功能中起着至关重要的作用。
3. 3. Researchers have been studying the propygidium for decades to understand its biomechanical properties.
   研究人员已经研究了这种结构几十年，以了解其生物力学性质。