The order consists of a single small family, Appendiculariidæ. The lowest type is known as Kowalevskia, a minute creature without heart or intestine found floating in the Mediterranean. It is in many respects the simplest in structure among Chordate animals. Oikopleura (Fig. 288) is another genus of this group.
Ascidiacea.—In the Ascidiacea the adult is usually attached to some object, and the two apertures are placed near each other by the obliteration of the caudal area. The form has been compared to a "leathern bottle with two spouts."
Fig. 280.—Ascidia adhærens Ritter. Glacier Bay, Alaska. (After Ritter.)
The suborder Ascidiæ simplices includes the solitary Ascidians or "sea-squirts," common on our shores, as well as the social forms in which an individual is surrounded by its buds. The common name arises from the fact that when touched they contract, squirting water from both apertures. The Ascidiidæ comprise the most familiar solitary forms, some of them the largest of the Tunicates and represented on most coasts. In the Molgulidæ and most Ascidiæ compositæ the young hatch out in the cloaca, from which "these tadpoles swim out as yellow atoms," while in a new genus, Euherdmania, described by Ritter, from the coast of California, the embryos are retained through their whole larval stage in the oviduct of the parent. They form, according to Kingsley, adhesive processes on the body, but those of Molgula cannot use them in becoming attached to rocks, since they are entirely inclosed in a peculiar envelope. This envelope is after a while very adhesive, and if the little tadpole happens to touch any part of himself to a stone or shell he is fastened for life. Thus "I have frequently seen them adhere by the tail, while the anterior part was making the most violent struggles to escape. Soon, however, they settle down contentedly, absorb the tail, and in a few weeks assume the adult structure."
In the family Cynthiidæ the brightly-colored red and yellow species of Cynthia are known as sea-peaches by the fishermen. The sea-pears, Boltenia, are fastened to long stalks. These have a leathery and wrinkled tunic, to which algæ and hydroids freely attach themselves. Into the gill-cavity of these forms small fishes, blennies, gobies, and pearl-fishes often retreat for protection.
Fig. 281.—Styela yacutatensis (Ritter), a simple Ascidian. Family Molgulidæ. Yakutat Bay, Alaska. (After Ritter.)
The social Ascidians constitute the Clavellinidæ. They are similar to the Ascidiidæ in form, but each individual sends out a bud which forms a stern bearing another individual at the end. By this means large colonies may be formed.