GKArrOLITES OF NEW YOUK, PAKT 1 . 525 



In the only case, Avhere the organ containing the siculae has been 

 observed, viz in Diplograptus foliaceus,it has been found that 

 these originate in cysts, which surround the base of the whole synrhabdosome 

 [p.528], and that they are partly retained, growing out into new rhabdosomes, 

 and partly discharged, producing new synrhabdosomes. Here the young 

 pass therefore apparently within vesicles through postembi-yonic stages, which 

 in the Hydroidea are passed only after fixation of the embryo. This is 

 evidently a secondary adaptation to the free planktonic mode of life of the 

 Axonophora. 



The fact that no such sicula-bearing vesicles have been found attached 

 to any of the frequently observed rhabdosomes of the Dichograptidae 

 would indicate that in these forms, which were pseudoplanktonic, the young 

 were discharged already in their planula stage, and the subsequent growth 

 stages ^vere passed after attachment to foreign bodies, as in the living 

 Hydroidea. 



The sicula has been termed the first theca by Holm. While Wiman's 

 investigations have taught us that the apertural part of the sicula, indeed, 

 has the form and function of a theca, it can also be inferred from his 

 work, that it still differs in essential characters from all later thecae of the 

 same rhabdosome. Thus in Diplograptus there is formed a solid axis, the 

 virgula, in the wall of the sicula, which is lacking in the other thecal walls, 

 and the aperture is provided with a long spine (the distal extension of the 

 axis) on one side, and with two lobes on the opposite. All of these charactei-s 

 fail to develop on the thecae. Elles's and Wood's and the writer's investiga- 

 tions have further demonstrated that the siculae frequently differ in their 

 dimensions from the thecae of the same rhabdosome. It is therefore to be 

 inferred that the first zooid, occupying the sicula, must have differed in 

 essential characters from the later individuals produced by gemmation. It 

 is, for this reason, appropriate to designate this first theca always by a special 

 term, viz sicula. 



The presence of a rod in one side of the sicular ^vall and that of the two 

 lobes on the opposite side of the aperture gives to the sicula a particularly 



