and Affinities of GraptoUtes. 367 



ance of the hydroid affinities of graptolites is found in the pre- 

 sence of a virgula, the rod or " solid axis/' which constitutes an 

 essential feature in the structure of the graptolite. This rod was 

 apparently of the same chitinous material as that which formed 

 the rest of the firm skeleton of the graptolite. It is frequently 

 continued for some distance beyond the distal or growing end, 

 while its opposite or proximal end usually terminates in a 

 minute spine ("radicle" of Hall), often continued into a 

 long slender filament, like that of the distal end. It grows 

 with the growth of the graptolite, as can be easily proved by 

 following the progress of the graptolite from its younger 

 stages ; and it is difficult to explain its increase of length and 

 thickness without regarding it, like the proper perisarc, as an 

 excretion from the coenosarc ; and though, in the adult grap- 

 tolite, it appears to ha^e been separated by a chitinous film 

 from immediate contact with the soft contents of the common 

 tube, it was probably in direct relation with these in the 

 younger stages, and would thus owe its existence to a special 

 activity and peculiar modification of the chitine-excreting func- 

 tion of the coenosarc at this part. It is sometimes found in the 

 single-rowed graptolites to have become detached from the 

 test or chitinous perisarc, leaving behind it a fm-row in which 

 it had lain, this furrow being, in the more perfect state of the 

 fossil, converted into a tube by a thin extension over it of the 

 test. 



Though the virgula would thus form an extremely excep- 

 tional structure, its presence can hardly be regarded as offering 

 an insurmountable obstacle to the admission of the graptolites 

 into immediate relation with the Hydroida. Until lately a 

 similar structure would have quite as justly excluded from the 

 Polyzoa any animal which possessed it., The discovery, how- 

 ever, of the living polyzoal genus Rhahdopleura shows that a 

 rod quite like that of the graptolite in all points, except in its 

 not being continued beyond the cell-bearing portion, might be 

 developed in an animal possessing in all other respects a typical 

 polyzoal structure*. 



It is true that the extension of the rod in the fossil beyond 

 the limits of the common tube appears to increase the diffi- 

 culty of reconciling its presence with the hydroid affinities 

 of the graptolite, I believe, however, that this is, after all, 

 not so anomalous a fact as at first sight it may appear, and 

 that there is reason to believe that the coenosarc invested by a 

 proper perisarc was originally continued along what now ap- 



* Allmau, "On Rhahdopleura,'" in 'Quarterly Joimial of Microscopic 

 Science,' Jan. 1809, p. .'57, pi. 8. 



