Structure and function of the locomotory system ofPolyorchis montereyensis (Cnidaria, Hydrozoa)

1. The structure and function of the locomotory system of the anthomedusanPolyorchis montereyensis Skogsberg were studied in detail. Anatomical investigations were carried out primarily on fresh or formalin fixed specimens; histology was done on specimens fixed in Bouin's fluid. Functional analyses were based largely on photography and cinematography. 2. Swimming inP. montereyensis involves the alternating antagonistic action of the subumbrellar swimming muscles and the elastic mesoglea. 3. The swimming muscle consists of striated contractile elements arranged circularly in four discontinuous subumbrellar sheets and a sheet on the subumbrellar side of the velum. There is also a sheet of radially arranged fibers on the exumbrellar side of the velum. The four subumbrellar sheets are anchored to the bell along the perand interradii. 4. The mesogleal skeleton consists of five components: (a) the matrix of the bell mesoglea, (b) optically visible fibers that traverse the bell from gastrodermal lamella to exumbrella, (c) the basement membrane or supporting lamella, (d) a system of joints, and (e) the velar mesoglea. 5. The morphology, orientation, and distribution of the mesogleal fibers suggest that their major role is maintaining the radial integrity of the bell during deformation. The amount of stretch in a region of the bell wall during contraction is inversely proportional to the number of fibers per unit area there. In regions of the bell which are not deformed during contraction fibers are sparse or absent. 6. Mesogleal volume remains constant during swimming. Locally the mesoglea is subjected to forces of stretch and compression, but the critical element in narrowing the bell involves bending or folding the mesoglea around a series of structural joints. The fulcrum of these joints is anchored to the exumbrella by concentrations of mesogleal fibers. The joints consist of eight adradial regions of highly deformable mesoglea lacking visible fibers. The regions are triangular in cross section and are separated from the remainder of the mesoglea (98–99 % of the total) by the gastrodermal lamella. A circular apical joint is also present. 7. Sequential changes in shape and position of the bell relative to a fixed grid during contraction and recovery were measured in order to determine such parameters of swimming as rate of contraction, rate of expulsion of water, change in bell velocity during contraction and recovery, momentum, etc. 8. The function of the velum was determined by cinematographic analysis of swimming animals both before and after removal of the velum. In normal swimming the velum serves mainly to constrict the aperture of the bell, thus increasing the velocity of expelled water, and hence increasing the force driving the medusa foward. Medusae swam with a greatly decreased velocity after velum removal. 9. Turning is accomplished primarily by asymmetrical contraction of the exumbrellar velar radial muscles, whereby the velar aperture is displaced to one side, water is expelled obliquely, and the bell turns toward that same side. The ability to turn was lost after velum removal. 10. Studies of the relationship between individual size and the various parameters of swimming inP. montereyensis show that: (a) the duration of the contraction phase of the swimming beat is roughly proportional to the square root of the subumbrellar circumference (or bell height); (b) smaller individuals swim faster relative to their bell height than do larger ones; (c) the velum is relatively better developed in small animals and plays a proportionately more important role during swimming.