64 



HYDROZOA. GRAPTOLITOIDEA 



important respects from the Calyptoblastea, e.g. in possess- 

 ing a virgula and sicula, in the diminution in size of the 

 hydrothecae towards the proximal end of the polypary, in 

 the hydrothecae being nearly always in contact, and in the 

 free communication which exists in most cases between 

 the hydrothecae and the common canal ; their development 

 is also different — in the graptolites each hydrotheca is 

 budded off from another hydrotheca, but in the Calypto- 

 blastea the new polyps are budded off from the ccenosarc. 



Fig. 19. Diplograptus pristis, from the Utica Slate, New York. 



V 2 

 X -3-. 



(After Ruedemann.) 



Further, the graptolites never form the much-branched 

 tree-like colonies which occur so commonly in recent 

 hydroids, and the graptolites are never firmly fixed by 

 any root-like structure corresponding to the hydrorhiza. 

 Some authors have considered that the graptolites 

 were free-swimming animals ; but it is very probable 

 that some, at any rate, were attached to sea-weeds or 

 other foreign objects by means of the virgula which, in 

 some genera comes off as a free thread from the point of 



