374 MR. J. D. MACDONALD ON THE PHYSIOLOGY OF THE 



closed system of Tetrustemma. On the otlier hand, if it can be shown that the sinus 

 system of the Beroldce 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 Brachlopod and the atrium of the Ascidian ; but this brilliant deduc- 

 tion originated with Professor Huxley, and was supported loy 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 Polyzoon and of the Bra- 

 cJiiopod 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 

 lacnncB described by Mr. Hancock as included within the stout sheath of the alimentary 

 canal in Brachlopod a correspond to the perivisceral sinus of Tmiicata, their homologue 

 appears to be absent in the Bohjzoa. 



I do not desire to sustain the accuracy of all the deductions drawn in the foUowiirg 

 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 

 llydroid Polyp to the Mollush, 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 

 confotmded 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 Bohjzoa and their lineal* allies — the Braclviopoda, 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 hsemal 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 Pohjzoa and Brachiopoda differ from Tmiicata. 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 

 transverse in the Polyzoa and Brachiopoda. Now, in an excellent example of what may be called an Operculated 

 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 being 

 not only perfectly homologous, but in every respect similar to the tentacula of the Polyzoa. All the figures extant of 

 those cerri (to which it would be far better to apply the word tentacula) are stiff and unnatural. Thus, richly 

 ciliated organs, endowed with exquisite sensibility, which curl upon themselves or start back as with a conservati'v e 



