A 'protective shell' around the larval cocoon of Cephalodiscus densus Andersson, 1907 (Graptolithoidea, Hemichordata)

摘要

Within the framework of the Italian XVII PNRA expedition (austral summer 2001-2002), several colonies of the Graptolithoidea (=Pterobranchia+Graptolithina) species, Cephalodiscus densus Andersson and C. hodgsoni Ridewood, were collected by trawl from Tethys Bay (Terra Nova Bay, Ross Sea). These organisms were maintained in aquaria for 2 weeks and their behaviour observed. The dissection of some colonies of C. densus allowed the reconstruction of the eggs' fate and the documentation of the formation of a protective shell around the cocoons of larvae settled within the "parental" colony encasement. The shell is produced by cementing size-selected debris, either of organic or inorganic origin, accumulated inside the encasement. This is the first documentation of such a structure within living Hemichordata and it helps to reinterpret some of the "resting structures" within zooidal tubes or thecae of particularly well-preserved fossil graptolites.