The microscopic structure of the leg muscle of the sea‐spider, Anoplodactylus lentus

This contribution fulfills the second part of a projected comparative microscopic study of voluntary muscle.The present aim is t o devote attention t o hitherto undescribed specimens.Leg and wing muscle of various insects has become classic for the demonstration of sarcoplasmic cross striation, and has furnished the chief basis for our conceptions of the detailed structure and the functional phases of voluntary striped muscle.Judging from the published illustrations of insect muscle, however, the leg muscle of the sea-spider offers by comparison a material of exceptional clearness; and for this reason alone it would seem to merit a detailed description, more especially since it furnishes in short longitudinal series a complete set of structural transition stages correlating with functional phases.The first study in this series pertained to Limulus muscle (Anat.Rec., vol. 10, no. 3, 1916; page 2lOl).A point of peculiar interest in this connection concerns the fact that both the Xiphosura and the Pycnogonids (Pantopoda) are usually placed in 'incertae sedis' in the current text-books of zoology.A quite general concensus of opinion considers them more or less closely related, and ranks them both between Crustaceans and drachnoids.Limulus muscle, however, is in appearance very much more like vertebrate than like insect muscle; while