Catenicellid Cheilostome Broyozoa. I. Frontal walls

Abstract

Twenty-two species from 16 genera in the Family Catenicellidae were studied by use of scanning electron microscopy combined with transmission electron microscopy and numerous light microscopy methods. The probably primitive species Costaticella solida was emphasized. The family is considered relatively homogeneous. Every known type of frontal wall and both known methods of ascus formation found among ascophoran Bryozoa occur in the family. All the types of frontal wall found can be derived from a spinose, pericystal frontal wall similar to that of some cribrimorph cheilostomes. Many wall types occur in combination in the same species, even on the same zoid. Several independent sequences of evolution of frontal wall types are seen. A proximal apertural notch or sinus evolved at least four times. Ascopores are present in at least three genera; they represent persistent lacunae. Cryptocystal components of the frontal wall evolved at least four times from four different structures; twice as incomplete, imperforate ledges and twice from floors of coelomic chambers provided with communication pores. Umbonuloid frontal walls are represented by calcified ascus roof overlain by uncalcified areas of gymnocyst. Primitive catenicellids possess one uncalcified window per spine. Spines tend to be lost by fusion and shortening; windows may be least, multiplied or enlarged. Reduction and loss of the spinose area (pericyst) was accompanied by expansion of the gymnocyst. In many catenicellids spines are reduced to a pair of folds of body wall which fuse just proximal to the aperture. In a few species, including all vittate forms, vestiges of spines are lost. Umbonuloid ascus formation is associated with pericysts, gymnocysts and sometimes umbonuloid frontal walls, and is always accompanied by some subsequent lepralioid ascus formation. Umbonuloid asci probably preceded lepralioid asci during evolution. Some species, including vittate forms, have exclusively lepralioid ascus formation. Lepralioid ascus formation is associated with gymnocysts, cryptocysts and most umbonuloid frontal walls. These findings suggest that many of the frontal wall morphologies found among ascophoran cheilostomes may have evolved independently. Higher taxa based exclusively on frontal wall type may be artificial. Some aspects of the ultrastructure, development and functional significance of the frontal wall are discussed.