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The results reported here provide a detailed account of the general morphology and histology of the male and female gonads of the sea-star Asterias vulgaris. The reproductive system of this sea-star (normal five-rayed specimens) consists of 10 separate units, each located proximally on the lateral wall of the ray, one on either side of the ray. Each unit is composed of a gonad, a gonoduct, and genital branches of the aboral haemal and coelomic rings. The gonad is a single bag-like structure with several protrusions (termed major and minor acini) extending from its surface. Its wall is composed of two sacs, one inside the other, separated by the genital coelomic (perihaemal) sinus. The outer sac consists of visceral peritoneum, an elastic-collagenous connective tissue layer, and many epithelial cells and circular muscle fibers. The inner sac comprises epithelial cells and longitudinal muscle fibers, the haemal sinus and contents, and germinal epithelium. The haemal sinus includes the haemal sinus space, filled with granular haemal fluid, cells, and collagen and other fine fibers enclosed by two fibrous laminae. Significant modifications in the form of the gonad and in the condition and relationships of the tissues which compose it occur during the annual reproductive cycle. Both sacs are stretched during growth of the gonad, the outer layers becoming attenuated and the inner layers being pressed against the outer, often obliterating the genital coelomic (perihaemal) sinus. The inner group of tissues is often extensively folded, pushing ridges formed from the inner wall of the haemal sinus and germinal epithelium into the lumen of the gonad. It is pointed out that previous studies on the gonads of asteroids have been relatively few, with no study for any species dealing comprehensively with morphological and histological details of the gonad based on both light and electron microscopy. Comparison of the results of the present study with observations of previous investigators indicates that although significant differences occur (especially in morphological terms), the general features of the histology of the wall of the gonad of many sea-stars are similar. In order to broaden our base for comparative studies, and to pursue significant problems, morphological, histological, histochemical, and ultrastructural investigations should be extended to as many asteroid species as possible.

STUDIES ON THE REPRODUCTIVE SYSTEMS OF SEA-STARS. I. THE MORPHOLOGY AND HISTOLOGY OF THE GONAD OF ASTERIAS VULGARIS