426 GENERAL CHARACTERS OP TUNIC AT A. 



vibration of the cilia which cover the aerating surface. It is from 

 this constant action, that the unattached species appear to derive 

 the slight amount of independent locomotive power which they 

 possess.* In these, the two orifices are usually at the opposite 

 extremities of the body ; and the continual suction of water into 

 one end, and the discharge of it from the other, will of course 

 tend to propel the body forwards. This movement is most 

 evident when several are associated together, all having their 

 branchial orifices and funnels in the same direction. In the 

 Pyrosoma ( 983), a number adhere together so as to form a 

 tube closed at one end, into the interior of which the funnels of 

 all the individuals open; whilst the branchial orifices project 

 from the outside as so many little papillae. The water drawn in 

 through these is discharged into the central canal ; from the end 

 of which a constant stream issues, with sufficient power to cause 

 the movement of the mass in the opposite direction, a move- 

 ment which its brilliant phosphorescence allows to be watched 

 from some distance. 



975. Although the Tunicata have been variously placed by 

 different Naturalists, the additions which have been recently made 

 to our knowledge of their organisation leave but little doubt, that 

 their true place is on the border of the Sub-Kingdom Mollusca, 

 connecting it with the Radiata. For, whilst its higher species 

 present many points of resemblance to the lower forms of the 

 Conchifera, its inferior tribes approach equally closely to the 

 Polypifera not only through the structure of the individuals, 

 but in the examples they present of the union of a number of 

 independent beings to form a compound animal. So prevalent, 

 indeed, is the tendency to this association, that it may almost be 

 regarded as the peculiar character of the group ; and, when thus 

 viewed, it presents a very good illustration of the general principle 

 formerly laid down ( 42, 43). We may regard the Tunicata, 

 then, as an aberrant group of Mollusca ; engrafting, as it were, 

 upon the general character of that Sub-Kingdom, the peculiar 

 tendency of the group of Radiata towards which it verges. The 

 tendency to aggregation exhibits itself among the Tunicata in 

 various ways. Sometimes we find a number of individuals 



