33 



mantle and these caecal tubuli ; and I carefully sought for evidence 

 of such a structure. In this, however, I was entirely unsuccess- 

 ful ; for the mantle, when stripped from the shell, presented no ap- 

 pearance whatever of having transmitted any such prolongations 

 into its substance ; on the contrary, it was evidently continued over 

 the mouths of the caeca with which it was in apposition ; and I fre- 

 quently found its external surface (that in contact with the shell) 

 covered in patches with cells exactly resembling in size and aspect 

 those contained within the caeca. I was equally unsuccessful in the 

 attempt to trace any other connection between these caeca and the 

 soft parts of the animal ; so that, although their importance in its 

 oeconomy scarcely admitted of doubt, the nature of their function re- 

 mained entirely unknown. The idea that they had any connection 

 with the formation of the shell itself, seemed to be completely nega- 

 tived by the fact, that in a large proportion of the group of BRACHIO- 

 PODA, no such perforations exist ; notwithstanding that their shells, 

 in every other feature of minute structure, are exactly accordant 

 with that of Terebratula. The foregoing results were communicated 

 to the British Scientific Association in 1847, and were embodied in 

 the Second Part of my " Report " published in its Transactions for 

 that year. 



The physiological importance of the characters of ' perforation ' 

 or ' non-perforation ' has become continually more obvious, as the 

 principles on which the subdivision of the group of Brachiopoda 

 should be founded, have been gradually settled by those who have 

 concerned themselves with its systematic arrangement ; and in par- 

 ticular, the universal presence of the perforations in the shells of the 

 family Terebratulidee, contrasted with their equally universal absence 

 in those of the family Rhynchonellida, unequivocally marked its rela- 

 tion to the general conformation of the animals of these subdivisions. 



Having been requested by Mr. Davidson to undertake a more de- 

 tailed investigation than I had yet made, into the minute structure 

 of the shells of Brachiopoda, for the sake of throwing still further 

 light upon the classification of the group, I applied myself afresh 

 to the solution of the problem, and believing that I have succeeded 

 in ascertaining the import of this curious feature in the organization 

 of Terebratula and its allies, I beg to offer an account of my re- 

 sults to the Royal Society. 



