374 . MR. J. D. MACDONALD ON THE PHYSIOLOGY OF THE 
closed system of Tetrastemma. On the other hand, if it can be shown that the sinus 
system of the Beroidce is derived from the alimentary membrane, while that of the Bra- 
chiopod, like the atrium of the Tunicary, has an external origin, analogy alone is all that 
can be sustained in regard to them. 
"With the exception of their having an external outlet for the discharge of ova or their 
products, an ordinary observer would never dream of any homology existing between the 
pallial sinuses of the Brachiopod and the atrium of the Ascidian ; but this brilliant deduc- 
tion originated with Professor Huxley, and was supported by Mr. Hancock. The abso- 
lute necessity of tracing up the routine development of everything in order to determine 
philosophical homologies, owes much of the importance which it has assumed in this 
country to the teaching of the former gentleman ; but I am not aware that this test has 
been yet tried in the case under consideration. 
Though it may be affirmed that the perivisceral cavity of the JPolyzoon and of the Bra- 
chiopod correspond with each other, that of neither can be said to be homologous with 
the perivisceral sinus of the Tunicary ; nor can it be altogether denied that the nutritive 
function is more or less subserved by the part cited in all. Should the sinuses or 
lacuna described by Mr. Hancock as included within the stout sheath of the alimentary 
canal in Brachiopoda correspond to the perivisceral sinus of Tunicata, their homologue 
appears to be absent in the Polyzoa. 
I do not desire to sustain the accuracy of all the deductions drawn in the following 
summary, but I believe that the points requiring elucidation will be more distinctly 
perceived in an attempt of this kind than by presenting them in an isolated form. 
In tracing up the development of the digestive and circulatory systems from the 
Kydroid Polyp to the Mollusk, we find a gradually increasing division of labour in the 
play of the organic functions going hand in hand with a progressive complexity of struc- 
ture. In the Hydrozoa there is no very obvious distinction between the stomach and the 
common cavity of the body, and therefore the functions of digestion and of circulation are 
confounded to a corresponding extent. 
In the Actinozoa, on the other hand, a true stomach is developed within the common 
cavity, though a communication is still preserved between them. Another step leads us 
to the Polyzoa and their lineal* allies— the Brachiopoda, in all of which the communi- 
* The word lineal is here used to distinguish between the affinity existing between the two orders noticed and their 
relationship with the Tunicata, which is merely collateral or representative, partaking little of affinity. 
The remarkable homologies and resemblance of structure which present themselves when the Polyzoa are compared 
with the Brachiopoda, cannot be for a moment denied, from the original hajmal flexure of the intestine, as pointed out 
by Professor Huxley (and in which particular they depart from the Tunicata), to more minute anatomical details. In 
passing, I think I can mention a new point in which both Polyzoa and Brachiopoda differ from Tunicata. It has been 
well said that the line of the hinge of the valves is dorsal and longitudinal in Lamellibranchiata, while it is dorsal and 
Now, in an excellent example of what may be called an Operculate 
Ascidian in my possession, the articulation is transverse, but lateral. 
I must say that due stress has not been laid upon the characters of the cerri, so called, of the Brachiopod, as bein 
not only perfectly homologous, but in every respect similar to the tentacula of the Polyzoa. All the figures extant of 
... ~ rr -, — ..- „» m/ ^ ow and unnatural. Thus, ncnir 
ciliated organs, endowed with exquisite sensibility, which curl upon themselves or start back as with a conservative 
transv 
tentacula) are stiff 
