Communication organs (septulae) of cheilostome Bryozoa are more complex than perviously believed. Annuli, present only in lateral septulae, are thickenings of the intercalary cuticle. Each communication pore is filled with a ring-like "pore cincture," through which project a pair of "special cells." Septulae of all species examined (10 species from 6 families) can be considered modifications of the same structure, varying only in degree of calcification and number of communication pores.External walls, including basal and laternl walls, are best defined as reinforcements of the ectocyst, which is derived by intussusception from the primary cuticle of the ancestrula. The lateral ectocyst must be considered a double layer formed by invagination of the distal ectocyst. Internal walls are developed by apposition from inner parts of the ectocyst; they include pore plates and transverse walls.External walls are laid down first. Lenticular masses develop unilaterally an the uncalcified lateral ectocyst; the pore plate develops by apposition from the interior part of the ectocyst. Depending on the species, the pore plate may or may not be calcified at the time of its formation. Communication pores are formed when the developing pore plate abuts against embryonic special cells. The septular ectocyst never cab cifies; it breaks down when the pore plate is complete.Some ascophorans undergo "reparative budding," in which new zoids are formed within dead zoecia. Hollow, ectocyst-covered buds lined with blastemic epithelia are produced from septulae of live zoids; adjacent buds may fuse.These findings are consistent with the view that lateral septulae are aborted zoids and that pore plates represent transverse walls.Without exception, bryozoans are colonial. The association between individuals is most obvious in the Phylactolaemata, in which body cavities of adjacent zoids are confluent. In the Eurystomata, however, zoids are separated by septa pierced by holes plugged with specialized cells. Despite the fact that there is no actual confluence, these holes are called "communication pores."There is evidence that the cells associated with communication pores ("rosette cells") are involved in transport of materials from one zoid to another (Bobin, '64, '65).Communication pores differ in number, arrangement, and the morphology of accessory structures, but there are underlying similarities. The similarities were emphasized by Silen, who based much of his complex theory of evolution of eurystome Bryozoa on the structure and development of interzoidal communication organs. Silkn ('42, '44a, '44b) proposed that all J. MORPII., 129: 149-170Eurystomata were derived from a common ancestral form similar to the "protocheilo. stome,"' Labiostomella gisleni (Silkn). He emphasized budding patterns in eurystomes, especially as related to the structure of zoecial walls and communication organs. Siliin's theory requires that lateral communication organs in cheilostome are to be considered aborted zoid buds. As S i l h himself points ou...
