GENERAL CHARACTERS OF TUNICATA. 427 



current of water enters the branchial orifice, and is propelled 

 through the funnel, without any other physical agency that can. 

 be perceived, than the vibration of the cilia which cover the 

 aerating surface. It is from the contractions of the muscles of the 

 inner tunic, 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 ( 1065), 

 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 papillas. 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 movement which its brilliant phos- 

 phorescence allows to be watched from some distance. 



1059. Although the Tunicata have been variously placed by 

 different Naturalists, the additions which have been recently 

 made to our knowledge of their organisation leave no doubt that 

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

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

 present many points of resemblance to the lower forms of the 

 Conchifera, the 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 



