Spines in the hindgut of the crayfish Procambarus clarkii (Decapoda): Their distribution and correlation with hindgut muscles

Spinal distribution and form in the hindgut lumen were observed in the cray fish Procambarus clarkii with the scanning electron microscope. The morphol ogy of the circular and longitudinal muscles in the hindgut were observed b y light microscopy. A relationship between spinal distribution and hindgut movement was suggested. The spines were finer, longer, and positioned close ly together where the spongy reticular tissues and circular muscles had not developed, (1) At the anterior part of the hindgut, short thick spines wer e found on the tilelike cuticular surface of the anterior rectal pads. Dens ity was about 2 spines per 10 mu m(2). The spiny cuticle, the surface of wh ich was segmented into hexagonal or pentagonal tiles, covered the ridges of the rectal pads. A thin layer of circular muscles was observed at this reg ion of the hindgut. (2) At the middle part of the hindgut, the spines becam e longer and finer as they approached the anus. Density was about 7.5 spine s per 10 mu m(2) The thin layer of circular muscles remained unchanged. (3) At the posterior part of the hindgut, the spines were fine and short. Dens ity was about 3 spines per 10 mu m(2). Four to 6 layers of thick, well-deve loped circular muscles were observed. Spine density was measured using NIH Image. All spines pointed in the direction of the anus. From these results, we suggest that the spines are closely related to differences in the physi cal movement in various sections of the hindgut. This is indicated by the t hickness of the circular muscles and rectal pads in the respective regions.