CHORDATA 



CLASS UROCHORDA or TUNICATA 



THE most conspicuous animals of this class are the ascidians, 

 which are common objects on rocky coasts. The simple 

 ascidians are peculiar leathery, sac-shaped bodies which send out 

 jets of water when touched. This habit gives them the common 

 name of " sea-squirts." Some are highly colored, especially 

 those of Southern waters ; others are somber, unattractive bodies, 

 often growing in masses. The compound ascidians are gelatinous 

 colonies, sometimes forming thin incrustations, sometimes jelly- 

 like masses, on seaweeds, shells, etc. This class comprises also 

 the beautiful Salpa, a genus of free-swimming animals having 

 transparent bodies encircled by rings of muscular bands, and in 

 one stage forming chains of attached organisms which swim on 

 the surface of the sea and of bays. 



The tunicates are by some authors classed with the vertebrate ani- 

 mals because in the larval stage they have a uotochord ; this disap- 

 pears, however, in the adult form, and the animals are considered 

 degenerates. The tunicates are interesting to biologists from the 

 remarkable changes they undergo in their life-history, and (in some 

 genera) the marked phase of alternation of generation. One of 

 their curious anatomical features is the blood-vascular system. 

 The circulation is propelled by wave-like contractions of the 

 heart, which, after forcing the blood one way for a time, stops 

 and reverses the blood-current. The tunicates are widely dis- 

 tributed, and occur at all depths. (Plate LXXXV.) 



474 



