586 
MR. HUXLEY ON THE ANATOMY OF SALPA AND PYROSOMA. 
soma 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 Salpoe 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. 
Now in the genus Doliolum (88.)* * * § this epipharyngeal band has attained a much 
greater development (though the mouth still remains 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 those carried by the hypopharyngeal 
band, and hence there appear to be two branchiae, an upper and a lower'!'. 
But in Pyrosoma the mouth is on the ventral side of the animal ; the epipharyn- 
geal 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 i 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 Salpce and Pyrosomata. 
61. The Pyrosomata, in the main, have the closest similarity in structure to the 
Botryllidse and other compound Ascidians ; but in these latter, the separation be- 
tween the test and the outer tunic becomes more and more marked, until it attains 
its greatest amount in the Clavelinidse, Cynthise, &c. 
Now does this separation furnish a character of any value or importance, systema- 
tically ? Surely not, for the value of a character depends upon the number of differ- 
ences 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. 
Clavelhia and Perophora are acknowledged to be closely allied genera, and yet in 
the former the test and outer tunic are separated to their utmost extent ; in the 
latter:! they are as closely united as in any Salpa. 
In the Cynthiee the test and tunics are generally very distinct ; but in Cynthia 
ampulloides, judging by the descriptions of Van Beneden, they are confounded 
together §. 
Again, in the Salpa vaginata of Chamisso, the test makes its appearance as a 
* And it may be added in the genus Anchinaia, described in Wiegmann’s Archiv for 1833, ■which seems to 
be a most interesting transition form between the Salpa and Doliolum, if indeed it be not the young form of 
Doliolum caudatum itself. 
t See also on the homology of the branchial organs of the Salpa and ordinai'y Tunicata, M. Milne-Eb- 
WARDS, Sur les Ascidies composdes, p. 55. 
X See the very beautiful figures and descriptions of Lister in the Philosophical Transactions for 1834. 
§ Mem. de I’Acad. Roy. de Bruxelles, tome xx. 
