T. Davidson — Wliat is a Brachiopod ? 
263 
affinities of the Brachiopoda, or the exact position the group should 
occupy in the animal kingdom. The Invertebrata have been grouped 
into five sub-kingdoms, namely, the Frotozoa, Ccelenterata, Annuloida, 
Anmdosa, and Mollusca, and for many years the Brachiopoda 
have been considered to constitute a separate dass in the sub- 
kingdom Mollusca, a view still maintained by some distinguished 
naturalists. Milne-Edwards, some years back, separated the 
Mollusca into two divisions, Mollusca and Molluscoida, and into the 
last division he placed the Brachiopoda , Polyzoa, and Tunicata, an 
arrangement that has been followed by many naturalists. Although 
the greater number of zoologists have admitted the close connexion 
existing between the Polyzoa and Brachiopoda, some considerable 
doubt lias been expressed with respect to the affinities and position 
of the latter to the Tunicata. Moreover, a strenuous effort has been 
made by such excellent observers as Steenstrup, Morse, Kowalevsky, 
and Alex. Agassiz to demonstrate that the affinities of the Brachio- 
poda and Polyzoa are with the worms, and that they should form a 
division, or two divisions, of the Annulosa, and be placed close to 
the Annelids. 
In his review of Kowalevsky’s admirable memoir on the embryo- 
logy of Argiope, Thecidium, and Terebratula, Alex. Agassiz observes : 
“ The close relationship between the Brachiopoda and Bryozoa 
(Polyozoa) cannot be more fully demonstrated than by the beautiful 
drawings upon plate v. of Kowalevsky’s history of Thecidium. We 
shall now have at least a rational explanation of the homologies of 
Brach iopods ; and the transition between such types as Pedicellina to 
Membranipora and other incrusting Bryozoa is readily explained from 
the embryology of Thecidium. In fact, all incrusting Bryozoa are 
only communities of Brachiopods, the valves of which are continuous 
and soldered together, the flat valve forming a united floor, while the 
convex valve does not cover the ventral valve, but leaves an open- 
ing more or less omamented for the extension of the Lophophore.” 
With respect to the Tunicata we are reminded by Morse, that 
Kowalevsky, Kupffer, Schultze, and others, assign to them a position 
at the base of the Vertebrale series, through the affinities of some of 
their fortns to Arnphioxus, as well as their singulär embryological 
relations to the Vertebrates. Gratiolet States likewise that the Tuni- 
cata are in no way related to the Brachiopoda, and Hancock, in 1870, 
expressed himself to me as follows, “ Of late years I have gradually 
inclined to the opinion that the Brachiopoda are not so closely re- 
lated to the Tunicata as we at one time thought. I am now busily 
engaged in working out the Tunicata, and they seem to be very in- 
timately connected with the Lamellibranchs. I am disposed to 
consider that there is a considerable hiatus dividing the Tunicata 
from the Brachiopoda and the Polyzoa or Bryozoa, and that these 
two latter groups alone should be included in the Molluscoida. If 
therefore Morse can establish his doctrine, it will relieve me of some 
little difficulty, inasmuch, as it will militate against Huxley’s view 
that the branchial sac of the Ascidian is the homologue of the 
pharynx of the Polyzoa. My idea being that the branchial sac is the 
