. Bonner zoologische Monographien. Zoology. 34 stages of Branchiostoma lanceolatiim with nine segments do not show any cellu- lar specializations of excretory function. The anterior mesodermal segments are restricted to the dorsolateral sides of the animal; the mesodermal cells of this region are fairly unspecialized but of a clear polar organization and surround a relatively spacious coelomic cavity. Embryos with eleven segments show orga- nelles resembling those of excretory cells of the adults, with a central cilium sur- rounded by ten rod-like microvilli (Stach 1994, Stach & Eisler 199
. Bonner zoologische Monographien. Zoology. 34 stages of Branchiostoma lanceolatiim with nine segments do not show any cellu- lar specializations of excretory function. The anterior mesodermal segments are restricted to the dorsolateral sides of the animal; the mesodermal cells of this region are fairly unspecialized but of a clear polar organization and surround a relatively spacious coelomic cavity. Embryos with eleven segments show orga- nelles resembling those of excretory cells of the adults, with a central cilium sur- rounded by ten rod-like microvilli (Stach 1994, Stach & Eisler 1998). Only a sin- gle excretory cell could be detected in serial sections, situated in the second meso- dermal segment of the left series. On its visceral side the cell rests on the extra- cellular matrix covering the notochord. No podocytic extensions of this cell could be seen. A coelomic space lies lateral to this cyrtocytic cell. Again the mesoder- mal cells of this segment, including the cyrtocyte, are definitely polar in organi- zation. In larvae where the mouth and first gill slit are open, Hatschek's nephri- dium is fully developed. It is situated immediately behind the preoral pit just above the left-sided mouth opening (Fig. 15, 16; Plate 1 IB & C). The nephridium stretches over a length of approximately 40 ^m and measures some 15 jum at its broadest diameter. It is situated lateral to the caudal end of the first segment of the left series in the second segment and comprises of five different cell types (Fig. 16): - Dorsally next to the neural tube a small number of myocytes (about 5) can be seen. They possess many mitochondria. Myofilaments are an*anged in the typical striated manner of skeletal muscle cells. - The somatic side of the nephridium consists of extremely narrow epithelial cells separated from the ectodermal epithelium by a common extracellular matrix. - The extracellular matrix ventral to the notochord is thickened. A lighter, finer- granulated region
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