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 -denned 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 CHORD AT A, 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 



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