The ancestrula and early growth pattern in two primitive cheilostome bryozoans: Pyripora catenularia (Fleming) and Pyriporopsis portlandensis Pohowsky

Abstract

The ancestrula and primary zone of astogenetic change have been recognized for the first time in two primitive cheilostomes. In Pyripora catenularia the ancestrula is stout, has a relatively large operculum, and buds four periancestrular zooids which initiate distal, proximal and two lateral branches. Zooid length increases progressively through a primary zone of astogenetic change encompassing about two to five asexual generations of zooids. Proximally directed budding from fractured branches commonly gives the erroneous appearance of a twinned ancestrula without a succeeding zone of astogenetic change. In Pyriporopsis portlandensis the tiny ancestrula is probably an autozooid occluded by a closure plate in all examples found. The ancestrula buds two periancestrular zooids, one apparently positioned distally and the other proximally or proximolaterally. Zooid length increases through a zone of astogenetic change exceeding five asexual generations in extent. Early growth patterns in pyriporids resemble those of some species of Electra and Conopeum in producing a proximal periancestrular zooid.