Ostracods are small (rarely more than a few millimeters across) but widespread crustaceans. Ostracods are mostly head; the rest of the body is greatly reduced, bearing at most two pairs of appendages (fewer than in any other crustaceans), and showing no sign of segmentation externally. The body is completely encased within a partially calcified, bivalved carapace.Look! the adductor muscle in the center of its body!

Photo by William Tan

介型蟲分布廣泛，其實海水和淡水都有分布，但因為體型很小，拍黑水的時候看起來就跟沙粒一樣，所以很少人能拍到介型蟲在水下的原貌。介型蟲的頭部佔據了絕大部分，並有兩對明顯的附肢（比其他甲殼類要少），然後從外觀看不出身體有分節，身體被部分鈣化的雙片外殼包覆。看照片中，身體中央的閉殼肌！
拍攝： William Tan 老師