6. THE COELOM AND ITS DERIVATIVES - THE EXCRETORY SYSTEM 



65 



In Polybrackia annulata and Lamellisabella zachsi the somatopleura is 

 highly modified in the middle part of the trunk from the level of the zone of 

 thickened papillae to the beginning of the postannular region. It is grossly 

 thickened here, forming a very characteristic spongy tissue, not unlike a 

 parenchyma filling the whole body cavity (Fig. 45). In the peripheral zone 



Fig. 45. Transverse section of a male Lamellisabella zachsi somewhat in front of the girdles of 

 the metasoma. 



be - ciliated band of sperm sac ; cv - corpus cardiacum ; eps - epithelium of sperm sac ; gt - pyriform 

 gland; mes - mesentery; my - muscle cell-bodies (myocytes) of the vessels; nc - neurocord (giant 

 nerve fibre); nil - dorsal nerve trunk; per - peritoneum; ss - sperm sac; ts - spongy coelomic 

 tissue; vc - commissural blood vessel; vt/ - dorsal vessel; vv - ventral vessel. (After Ivanov, 1958a.) 



this tissue has numerous round or star-shaped cells with oval nuclei, joining 

 into an irregular network by means of their own cytoplasmic processes. The 

 cells which lie contiguous to the longitudinal musculature are bound up with 

 them into long bundles. Vacuoles devoid of nuclei are developed between the 

 cells. The central part of the body cavity is occupied by similar masses. What 

 function is performed by this highly distinctive tissue is unknown. It cannot 

 be doubted, however, that it is not an artefact but represents a modification 

 of the peritoneum (Ivanov, 1960a). 



