. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 194 M. SHIRAE ET B Figure 8. Hemocytes and vascular epithelium in the rejection type initiated before fusion of ampullae in Symplexma reptans. (A) Blood vessel and outside of blood vessel. (B) Disintegration of morula cell inside blood vessel. (C) Disintegration of infiltrating morula cell in tunic. Arrowheads indicate electron-dense liber in the tunic, m. infiltrating morula cell; t. tunic: ve. vascular epidermal cell. Bars = 2 tion of MCs in the tunic promoted disintegration of the tunic matrix. Phagocytosis occu
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 194 M. SHIRAE ET B Figure 8. Hemocytes and vascular epithelium in the rejection type initiated before fusion of ampullae in Symplexma reptans. (A) Blood vessel and outside of blood vessel. (B) Disintegration of morula cell inside blood vessel. (C) Disintegration of infiltrating morula cell in tunic. Arrowheads indicate electron-dense liber in the tunic, m. infiltrating morula cell; t. tunic: ve. vascular epidermal cell. Bars = 2 tion of MCs in the tunic promoted disintegration of the tunic matrix. Phagocytosis occurred in both types of rejection reaction, but encapsulated hemocyte clusters were not found. In the first type of rejection reaction (rejection before vascular fusion), tunic disintegration was limited to the regions surrounding the tips of ampullae in the contact area, and it was more intensive than in the second type (rejection after vascular fusion). However, MC disintegration and phagocytosis inside blood vessels was more common in the second type of rejection reaction than in the first. In the second type of rejection, electron-dense material that might have been discharged from MCs was often found in the vascular lumen (Fig. 9, arrowheads). This material was most abundant in the fused ampullae. The second type of rejec- tion reaction in 5. reptans progressed more slowly than the rejection ^action of B. scalahs. In both types of rejection reaction in v reptans, cellular junctions between the epi- thelial cells ui unpullar tips became loose, and some gaps appeared (arrow in Fig. 8A). Some 5. reptans colonies could exhibit both types of rejection reaction, but others showed only the first type (Fig. 10). The fusion experiments with the same pair of colonies were repeated several times and always showed the same results. Discussion In five botryllids, Botr\llus primigenus, Botryllus schlos- seri, Botrylloides simodensis, Botrylloides fuscus, and Bot- rylloides violaceiis, the
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