2 
398 MR. HANCOCK ON THE ANATOMY OF THE FRESHWATER 
arranged in a circle about the mouth,—the other having them 
supported on two lateral oral arms. The former of these branches 
passes into the Ascidian,—the latter, I shall endeavour to show 
is connected with the Brachiopoda. 
This connexion is at once suggested by the resemblance that 
exists between the oral arms of Plumatella and Alcyonella and 
the characteristic brachial organs in the Brachiopoda, particu- 
larly of those in Lingula. In both the Brachiopoda and the 
Bryozoa, the arms rise from the sides of the mouth in the same 
manner, the bases of the arms being confluent; and the tentacles 
or cirrhi forming a continuous series. In both the arms are 
hollow, and the tentacles and cirrhi are tough and non-contractile ; 
and in both they are prehensile organs after the same fashion. 
The digestive organs of both are very similar; and the whole of 
the Brachiopoda are fixed, and so are the Bryozoa, with but one 
exception. 
But what is still more remarkable, the muscular systems of 
both are arranged much in the same manner, particularly as re- 
spects Zerebratula and Paludicella with most of its marine con- 
geners. In Yerebratula, as the animal is fixed within the shell, 
of course there can be nothing resembling the polype retractors ; 
but the shell muscles of Zerebratula will be found to work exactly 
on the same principle as those provided to draw in the margins 
of the cell-orifice in Paludicella and Bowerbankia, and called by 
Dr. Farre, opercular muscles. 
There are four sets of muscles in connexion with the shell in 
Terebratula chilensis as dissected by Owen, two from each valve ; 
and they all pass diagonally downwards, and with one exception 
go to be inserted in the pedicle; so that when they contract the 
valves will be closed. These muscles then have in fact their 
origin in the pedicle, as stated by Owen, and acting from thence 
upon the moveable points of their insertions, operate precisely 
in the same way as the tube-retractors of the polypes last men- 
tioned do on the lips of the orifice. The action is the same in 
both; and were the cell-walls of Bowerbankia, for instance, cal- 
cified and divided longitudinally into two portions or valves, they 
would be made to close just as the valves do in Zerebratula. 
