70 ORIGIN OF CHORDATES m. 6- 



animals are perhaps all related, but the whole subphylum is con- 

 veniently subdivided into three classes. 

 Class i. Ascidiacea. 



Typical bottom-living forms such as Ciona (solitary), Botryllus 

 (colonial). 

 Class 2. Thaliacea. 



Pelagic forms, simple or colonial, swimming by means of circular 

 muscle bands. Salpa, Doliolum, Pyrosoma. 

 Class 3. Larvacea. 



Pelagic tunicata without metamorphosis; the adult has a tail and 

 resembles the tadpole of the other groups. Oikopleura. 



7. Class Ascidiacea 



The typical sessile ascidians are found in all seas. They may be 

 divided into those that live as single individuals (Ascidiae simplices) 

 and those forming colonies (Ascidiae compositae). Both types include 

 many different forms, however, and the division is not along phylo- 

 genetic lines. The colonial forms produced by budding may consist 

 simply of a number of neighbouring individuals {Clavelina) or of a 

 common gelatinous test in which the individuals are embedded 

 {Botryllus, Amaroucium). The form of the body is related to the type 

 of bottom upon which they are found; there has thus been an adap- 

 tive radiation within the group; a great variety of habitats is avail- 

 able for bottom living creatures, and the animals become adapted 

 accordingly. 



Most of the species live in the littoral zone, but a few deep-sea forms 

 are known, such as Hypobythius calycodes, found below 5,000 metres. 



Many ascidians probably live only for a short time, becoming 

 mature in their first year and dying thereafter. In some species the 

 animals live over a second winter, during which they become reduced 

 in size, growing and budding again in the following spring (Clavelina). 



8. Class Thaliacea 



These are pelagic tunicates living in warm water. They have 

 circular bands of muscle, enabling the animal to shoot through the 

 water by jet propulsion. In Doliolum and its allies the muscle-bands 

 pass right round the body (Cyclomyaria), whereas in Salpa the rings 

 are incomplete (Hemimyaria). The mouth and atriopore are at 

 opposite ends of the body. The tunic is thin and, like the rest of the 

 body, transparent. 



The life-history of these forms involves a remarkable alternation of 



