ASCIDIANS AND AMPHIOXUS
BY
W. A. HERDMAN, D.Sc. (Edinb.), F.R.S.
Professor of Natural History in the University of Liverpool
CHAPTER II
TUNICATA (ASCIDIANS AND THEIR ALLIES)
INTRODUCTION—OUTLINE OF HISTORY—STRUCTURE OF A TYPICAL ASCIDIAN—EMBRYOLOGY AND LIFE-HISTORY
The Tunicata are marine animals found in practically all parts of the sea, and at all depths. They extend from the Arctic and Antarctic regions to the tropical waters, and from the littoral zone down to the abyssal depths of over three miles. They are abundant in British seas. They vary greatly in shape and colour, and range in size from an almost invisible hundredth of an inch to large masses a foot or more in diameter. And yet most Tunicata have a characteristic appearance by which they can be readily distinguished from other animals. They form a well-defined group, with definite anatomical characters, and there are no known forms intermediate between them and other groups. The Tunicata were formerly regarded as constituting, along with the Polyzoa and the Brachiopoda, the Invertebrate Class "Molluscoidea." They are now known to be a degenerate branch of the lower Chordata, and to be more nearly related to the Vertebrata than to any group of Invertebrates.
Tunicata occur either fixed or free, solitary, aggregated or in colonies (see Fig. 27, p. [64]). The fixed forms, found on the sea-bottom, are usually termed "Ascidians," those that are solitary or merely aggregated being "Simple Ascidians" or Monascidiae, and those that are organically united into a colony being "Compound Ascidians" or Synascidiae. The colonies have been produced by budding, a process which is very general in the group, and the members of the colony are conveniently known as "Ascidiozooids." Some exhibit alternation of generations, and all pass through remarkable changes in their life-history, nearly all of them undergoing a retrogressive metamorphosis.
Outline of History.