SECTION VI 



TUNIGATES 



CHAPTER I 



STRUCTURE 

 General characters ; a typical solitary Tunicate. 



THE animals generally known as Tunicates or Ascidians, 

 more technically called Urochorda, occupy a remark- 

 able position in the animal kingdom. As adults, few 

 of them show any likeness to any other animal type, and the 

 affinities which a deeper knowledge reveals are in an unex- 

 pected quarter. Their relationships turn out to be with verte- 

 brates or backboned animals. But though the vertebrate 

 characteristics are indisputable, they are hardly recognisable ex- 

 cept in the larval stages, and in a few forms (Larvacea) which 

 retain their larval characters throughout life. In most cases it 

 must be said that the larva is an organism of higher degree 

 than the adult, for the promise of youth is generally unfulfilled. 

 To begin with, then, let us think of the Tunicates as a set of 

 animals that stumble at the threshold of vertebrate life. They 

 begin well, but few of them keep it up. As it is put technically 

 they usually undergo "retrogressive metamorphosis". 



Tunicates are found in all seas and at all depths ; most of 

 them are fixed forms, living as single individuals or in com- 

 pound masses formed by budding ; a few of them are adapted 

 for open sea (pelagic) life, and here again there are solitary 

 and compound forms. It is very interesting to find an animal 

 so high in the scale of being still experimenting with the pro- 

 cess of repeated budding, like plants or corals, a process which 

 is only compatible with the sessile, or passively floating con- 

 dition. 



A fine account of a typical solitary Tunicate will be found 



473 



