The Significance of Syncytial Tissues for the Position of the Hexactinellida in the Metazoa1

SYNOPSIS. Hexactinellid sponges are metazoans in which the major tissue component is a multinucleated syncytium. The preferred deepwater habitat of these sponges makes collection of hexactinellids in good condition difficult, and has hindered extensive examination of their body plan. Nonetheless, over the last three decades a number of studies have explored their ecology, histology and physiology. It has been shown that hexactinellids are extremely long-lived animals. Their cytoplasm consists of a giant, multinucleated tissue, the trabecular syncytium, which is connected via open and plugged cytoplasmic bridges to cells such as archaeocytes, choanoblasts, and cells with spherical inclusions. Because all of the sponge is cytoplasmically interconnected, electrical signals can propagate through the animal. The effector response is arrest of the feeding current. The perforate plugged junction apparently allows tissues to specialize in different ways while maintaining limited cytoplasmic continuity. Larvae of hexactinellid sponges are already largely syncytial. Although it is not known when the first syncytial tissues are formed or when perforate plugged junctions first appear during embryogenesis, evidence that embryos are cellular until gastrulation suggests that hexactinellid sponges may have evolved from cellular sponges and that syncytial tissues are not a primitive trait of the Metazoa.