142 THE FAUNA OF THE DEEP SEA 



and the microscopic appendicularias of the pelagic 

 plankton. 



Notwithstanding the apparent simplicity of their 

 adult structure, naturalists are now agreed that they 

 must be removed from the Mollusca, with which they 

 have hitherto been most frequently associated, and 

 placed in the group of the Vertebrata. It is the 

 study of embryology that has led to this unexpected 

 conclusion, for we find, when we study the larval forms, 

 that they possess both a notochord and gill-slits, two 

 features that are characteristic of the group of the 

 Vertebrata. 



The species of the group Perennichordata, which 

 includes all those Tunicates that possess a noto- 

 chord persistent through life, are chiefly pelagic in 

 habit, the little creatures, rarely more than two or 

 three millimetres in length, swimming or drifting 

 about with the sagittas, copepods, ctenophores, and 

 medusae that compose the pelagic plankton. Fol has 

 recently described a gigantic form belonging to this 

 group, reaching a size of thirty millimetres in length, 

 called Megalocercus abyssorum, which he dredged 

 from a depth of 492 fathoms ; and other species have 

 been recorded down to a depth of 710 fathoms in the 

 Mediterranean Sea. 



