CirAde TAR. tl 
TUNICATA (ASCIDIANS AND THEIR ALLIES ) 
INTRODUCTION——OUTLINE OF HISTORY 
TYPICAL ASCIDIAN 
STRUCTURE OF A 
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 “ MoLLuscompEa.” 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 ageregated 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 
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