THE PHYLUM CHORDATA 



49 



genera and only a few species exist, the urochordates constitute a 

 large assemblage of highly diverse forms, ranging from purely pelagic 

 to purely sessile forms, and from solitary fonns of large size to colo- 



iTapound 

 cidians 



Fig. 21. — Sketch of the chief kinds of Urochordata found in the sea showing 

 their distribution and habits. The dotted hnes on the left indicate the Ufe zones 

 of the sea: the surface or pelagic zone; the middle zone; and the sea-bottom zone. 

 (From Herdman in the Cambridge Natural History, Vol. VII.) 



nial forms of comparatively small size. They are plentiful all over the 

 oceans from the shallow shore waters to the deepest abysses. By far 

 the majority of them are sessile in habit in adult life, remaining 



