J. nOPKINSON ON BRITISH GRAPTOLITES, 161 



doubt. Let us compare them with the three classes into which 

 zoophytes are divided — the Cajlenteratc Hydrozoa and Actinozoa, 

 and the MoHuscan Polyzoa. 



The Actinozoa are divided into two sub-classes — Alcyonaria 

 and Zoantharia. The Zoantharia have not the slightest resem- 

 blance to the graptolite. 



The Alcyonaria have a certain superficial resemblance ; the chief 

 point relied upon by those who refer graptolites to this sub-class 

 being the presence of a solid axis in certain genera — Pennatula, 

 Virgularia, SfC. ; but this axis is " thick and calcareous, and pro- 

 ceeds from the proximal end," while in the graptolite it is " slender 

 and corneous, and is produced at the distal end." The polypites, 

 moreover, are contained in the fleshy body of the Alcyonarian 

 zoophytes, while in graptolites they were contained in specially 

 developed thecse. 



Graptolites, therefore, have a totally different structure to the 

 Actinozoa. Let us try the Polyzoa. 



Most of the Polyzoa are protected by a calcareous polypary. In 

 graptolites it is chitinous. In most of them, also, each polypite is 

 entirely cut off from all the others by a septum. In one order 

 only — Ctenostomata, is there a chitinous polypary, and it is singular 

 that in this order the septum is not entire ; there is a small perfora- 

 tion by means of which the polypites are organically united to a 

 common canal. In graptolites, however, there is not the slightest 

 constriction, or indication of a dividing septum, if we except a few 

 forms in which there is an impressed line between the hydrothecse 

 and the periderm. This appears to be merely a surface mark, 

 similar to that at the base of the hydrothecse in the Sertulariadas. 



Graptolites, therefore, having no septum, either entire or per- 

 forated, cannot be Polyzoa. We are now reduced to the humbler 

 Hydrozoa. 



The polypary of many of the Hydrozoa is of the same nature as 

 that of the graptolite, and the internal structure is similar, as we 

 have already seen. There are free and fixed forms in the Hydrozoa, 

 and most probably, also, in the Graptolitid^e. It is true that in only 

 one genus of graptolites, Dendrograptus, have we an indication of a 

 hydrorhiza capable of attachment, while all the most nearly allied 

 Hydrozoa are fixed. 



The organic connection, without any septum, between the indi- 

 vidual polypites and the common ctenosarc, which is universal in 



