The primary purpose of this research is to further morphological studies on larval ascidians and to encourage greater reliance on larval structures in ascidian taxonomy. This may necessitate some revisions in current classifications, but it is hoped that such revisions will lead to a better understanding of the group. Some general considerations of ascidians are here included for the benefit of those who may not be familiar with them. All specimens used in this study were collected at the Pacific Marine Station area at Dillon Beach, California, during June, July, and August of 1957.