HYDROZOA. GRAPTOLITOIDEA. 45 



in many respects the gonangia of living Sertularians, 

 and very probably represent them. In the latter the 



FIG. 9. Diplograptus whitfieldi, with reproductive sacs. (After Hall.) 



gonangia are chitinous sacs, within which in the calypto- 

 blastic hydroids 1 , the gonophores are developed, and these 

 give rise directly to the generative elements. The earliest 

 condition of the graptolite at present known, is the sicula 

 (fig. 8 h), this probably arises indirectly from the sac-like 

 bodies which have been supposed to be gonangia. The 

 first change which takes place in the sicula, is the forma- 

 tion of the virgula in the wall of one side ; this frequently 

 projects beyond one or both ends (fig. 8 i). Next there 

 appears a bud on one face of the sicula, frequently near 

 its broader extremity but sometimes near the point ; 

 this developes into the first hydrotheca, and is soon 



1 These include such genera as Plumularia, Sertularia and Campanu- 

 laria. 



