6o ANATOMY AND PHYSIOLOGY OF SALPA AND PYROSOMA 



The inner membrane is just as much or as little adherent to the 

 outer in Pyrosoma as in Salpa. In each case the wide sinuses 

 between the two membranes form the sole vascular system. 



60. It may be said that there is an essential difference between 

 Salpa and Pyrosoma in the structure of the branchiae. A little 

 consideration, however, will show that this is merely a difference 



in degree. 



Savigny has shown that in certain Salpce there is an upper division 

 of the " gill," an " epipharyngeal band " (to carry out the nomenclature 

 adopted at (10)), as well as a hypopharyngeal band. 



Xow in the genus Doliolum (88ji this epipharyngeal band has 

 attained a much greater development (though the mouth still re- 

 mains at the upper part of the cavity), and like the hypopharyngeal 

 band carries a number of ciliated branchial bars. These bars have a 

 direction more or less parallel to tho.se carried by the hypopharyngeal 

 band, and hence there appear to be two branchia;, an upper and a 

 lower.- 



But in Pyrosoma the mouth is on the ventral side of the animal ; 

 the epipharyngeal band, developed in proportion to the distance of 

 the mouth from its normal position, takes a direction at right angles 

 to the axis, and thence comes to carry the branchial bars belonging to 

 it parallel to the axis ; while the hypopharyngeal band carrying its 

 branchial bars as before, the two sets cross and produce the branchial 

 network. 



The line between the Monochitonida and Dichitonida then can 

 certainly not be drawn between the Salpcv and Pyrosomata. 



61. The Pyrosomata, in the main, have the closest similarity in 

 structure to the Botryllid^ and other compound Ascidians ; but in 

 these latter, the separation between the test and the outer tunic 

 becomes more and more marked, until it attains its greatest amount 

 in the Clavelinidae, Cynthise, &c. 



Xow does this separation furnish a character of any value or import- 

 ance, systematically? Surely not, for the value of a character 

 depends upon the number of differences of which it is a mark ; and 

 this is the mark of none. 



Savigny observed the close resemblance between Botryllus and 

 Pyrosoma, which yet differ in this character. 



' And it may be added in the genus Aiuhinaia, described in Wiegmann's Archivfor 1833, 

 wtiich seems to Ije a most interesting transition form between the Salpa and Doliolum, if 

 indeed it be not the young form of Doliolum cattdatuijt itself. 



^ See also on the homology of the branchial organs of the SalpiC and ordinary Tunica/a, M. 

 Milne-Edwards, Sur les Ascidies composees, p. 55. 



