Structural Organization of the Blood‐sinus Systems in Lampreys and Hagfish: Functional and Evolutionary Interpretations

Abstract

Abstract The head and branchial regions of larval and adult lampreys and hagfish were studied histologically in serial sections. The most remarkable feature in these extant agnathans was the occurrence of large blood‐sinuses. In larval lampreys, blood‐sinuses are well developed in the velum, an organ that functions to introduce water and accompanying food particles from the mouth into the gill and alimentary regions. The sinuses in the velum may act to transduce the force of contraction of velar muscles to the stroke‐like movement of the velum; without these sinuses muscular contractions might simply cause the velum to collapse. In adult lampreys, blood‐sinuses are well developed in the peribranchial space that surrounds the branchial (gill) sac and is surrounded by the branchial pouch. It is possible that the force of contractions of the branchial‐pouch muscles is transduced effectively to the branchial sac via the peribranchial blood‐sinus and facilitates the expiration of water through the external gill pores. If the peribranchial sinus were absent, the muscular contraction might deform the branchial sac in an inappropriate manner. In the hagfish, the blood‐sinus system is also well developed in the velum and peribranchial space, although the peribranchial sinus lies outside the muscular branchial pouch. In agnathans, the blood‐sinus system may function, at least in part, as a kind of hydrostatic skeleton that transduces the force generated by muscular contraction.