The suborder, Ascidiæ compositæ, contains the compound Ascidians or colonies enveloped in a common gelatinous "test." These colonies are usually attached to rock or seaweed, and the individuals are frequently regularly and symmetrically arranged. The bodies are sometimes complex in form.
Fig. 282.—Styela greeleyi Ritter. Family Molgulidæ. Lukanin, Pribilof Islands. (After Ritter.)
Fig. 283.—Cynthia superba Ritter. A Tunicate from Puget Sound. Family Cynthiidæ. (After Ritter.)
In the Botryllidæ and Polystyelidæ the individuals are not segmented and in the former family are arranged in star-shaped groups about a common cloaca, into which the atrial siphons of the different individuals open. The group springs by budding from the tadpole, or larva, which has attached itself to some object. These forms are often brightly colored. Botryllus gouldi is a species very common along our North Atlantic coast, forming gray star-shaped masses sometimes an inch across on eel-grass (Zostera) and on flat-leaved seaweeds. Goodsiria dura, a representative of the Polystyelidæ, is one of the most common Ascidians on the California coast southward, where the brick-red masses incrusting on seaweeds of various kinds, and on other Ascidians, are frequently thrown ashore in great quantities during heavy storms.
Fig. 284.—Botryllus magnus Ritter. A compound Ascidian. Shumagin Islands, Alaska. (After Ritter.)
In Didemnidæ the body is more complex, of two parts, called the "thorax" and "abdomen." In Amarœcium, the "sea pork" of the fishermen, the body is in three parts and the individuals are very long. These sometimes form great masses a foot or more long, "colored like boiled salt pork, but more translucent." Other families of this type are the Distomidæ and the Polyclinidæ.