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 crayfish Procambarus clarkii with the scanning electron microscope. The morphology of the circular and longitudinal muscles in the hindgut were observed by light microscopy. A relationship between spinal distribution and hindgut movement was suggested. The spines were finer, longer, and positioned closely together where the spongy reticular tissues and circular muscles had not developed. (1) At the anterior part of the hindgut, short thick spines were found on the tilelike cuticular surface of the anterior rectal pads. Density was about 2 spines per 10 µm2. The spiny cuticle, the surface of which was segmented into hexagonal or pentagonal tiles, covered the ridges of the rectal pads. A thin layer of circular muscles was observed at this region of the hindgut. (2) At the middle part of the hindgut, the spines became longer and finer as they approached the anus. Density was about 7.5 spines per 10 µm2. The thin layer of circular muscles remained unchanged. (3) At the posterior part of the hindgut, the spines were fine and short. Density was about 3 spines per 10 µm2. Four to 6 layers of thick, well-developed 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 physical movement in various sections of the hindgut. This is indicated by the thickness of the circular muscles and rectal pads in the respective regions.