54 HOMOLOGIES. 



indicate liomological identit)^ though we should be cautious in concUxding as to homological 

 relations from mere resemblance ; where functions of the same kind are to be performed, 

 there must necessarily be a certain degree of resemblance in the instruments, and this resem- 

 blance may show itself not only in form but in position and relation to surrounding parts, 

 though the organs themselves be fundamentally different ; and it needs the light afforded by 

 development before our conclusions as to homological identity can be considered as fully 

 established, while our ignorance of facts connected with the embryology of the Brachiopoda 

 deprives us of this important test in the present instance. 



Mr. Huxley* has thrown still further light on the relations between the Polyzoa and 

 Brachiopoda. He compares the relative positions and mode of articulation of the valve which 

 closes the cell in the marine cheilostomatous Polyzoa and of the peculiar bodies known as 

 avicularia, which occur in the same group of Polyzoa, with corresponding characters in the 

 shells of the typical Brachiopods. He shows that the smaller valve of the Brachiopod is 

 articulated with the larger, just as the operculum of the Polyzoon is united with its cell, or as 

 the lesser valve of an avicidarium is articulated with the larger one ; while the anus has the 

 same relative direction in both, being in the one case turned away from the operculum of the 

 Polyzoon, and in the other from the smaller valve of the Brachiopod. He further shows that, 

 leaving the pedicle muscles out of consideration, the arrangement of the other muscles is 

 exactly what occurs in an avicularivm ; " the adductors, which pass from the larger valve to 

 be inserted into the smaller, in front of its point of support, corresponding precisely with the 

 occlusor muscles of the avicularium ; while the cardinal muscles which arise from the larger 

 valve, and pass to be inserted into the cardinal process of the smaller, behind the point of 

 support, are identical with the divaricator muscles of the avicularium." 



The relations involved in the above views are very interesting. In order, however, to 

 admit them, even to the extent which our ignorance of Brachiopodal embryology will admit, 

 we must cease to look upon the avicularium as a mere organ in the ordinary sense of this word, 

 and consider it as a distinct though very peculiar zooicl, homologically repeating the structure 

 of the ordinary zooid, a view against which no valid objections can be urged. 



We thus perceive that the affinities of the Polyzoa pass off in two very evident directions 

 — in one through the Tunicata, and in the other through the Brachiopoda ; while the Tunicata 

 and Brachiopoda conduct us by well-marked relations through the Lamellibranchiata into the 

 higher MoUusca. 



In determining the limits between the Mollusca and the Mollmcoida, Huxley draws the 

 boundary line upon the molluscan side of the Brachiopoda, thus including the latter in the 

 Molluscoida. To this view, however, I have not been able to assent. Notwithstanding the 

 obvious affinities just referred to between the Bracldopoda and the Poh'zoa, I yet believe that 

 the Brachiopoda are much more intimately allied to the Lamellibranchiata than they are to the 

 Polyzoa. 



The oral arms of the Brachiopoda, in which the most interesting resemblance between 

 this group and the Polj'zoa is to be found, while they are undoubtedly related to the lophophore 

 of the latter, have just as decided a relation to the labial tentacles of the Lamellihranchiaia , 

 while not only in the condition of the nervous and circulatory systems, which are both upon 



* Article " JloUusca" in ' English Cyclopsedia,' 1855. 



