MANY SQUIRTS, PACKED TOGETHER—Some species of sea squirts appear to be one large organism, but when viewed under a microscope, it is evident that they are actually a colony made of hundreds and thousands of tiny, linked "squirts," called zooids. Here, two species of sea squirts grow next to each other. The small, orange sea squirts are of the genus Botrylloides. The whitish, larger squirts are of the genus Didemnum. (Photo by Dann Blackwood, USGS)
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