64 



PART I. GENERAL ACCOUNT 



Fig. 44. The wall of the dorsal vessel of 

 Spirobrachia grandis in transverse section. 

 ce - peritonal cell, containing excretory 

 inclusions ; fin - contractile fibres of 

 myocytes; int - intima of the vessel; 

 my - muscle cell-body (myocyte) and 

 nucleus; sa - blood. (After Ivanov, 

 1960a.) 



The walls of the mesocoele form a 

 peritoneal epithelium which is particu- 

 larly apparent on the surface of the 

 dorsal blood vessel and its blind lateral 

 outgrowths. The peritoneal cells here 

 are tall and large, and as a rule are 

 filled with numerous excretory granules 

 so that they may be regarded as 

 chloragogenous cells (Fig. 44). The 

 internal surface of the longitudinal 

 musculature is lined by a more or less 

 broken layer of round cells which are 

 attached by a narrow constriction or 

 at the base (Fig. 30). A thin layer of peri- 

 toneal cells covers the surface of the 

 multicellular flask-shaped glands which 

 lie in the coelom of the mesosoma. 



The mesocoele is divided from the 

 colelomic sacs of the metasoma by a 

 well-developed transverse muscular 

 diaphragm (Johansson, 1937; Ivanov, 

 1955a). 



The metacoele 



The coelomic sacs of the trunk — the 

 metacoeles — extend the whole length 

 of the third segment. As with the coeloms of the mesosoma they are divided 

 from each other by a more or less muscular mesentery, but a little in front of 

 the girdles the mesentery disappears between the dorsal and ventral blood 

 vessels for a considerable length so that the right and left coelomic sacs are in 

 communication with one another. At the level of the girdles a special median 

 cavity begins, formed from the walls of the coelom (Figs. 22, 46). 



The walls of the metacoele are clad with a peritoneum which in the fore- 

 part of the trunk is generally similar to that of the mesosoma. The splanchno- 

 pleura has a particularly tall and distinctive epithelium. On the surface of the 

 dorsal vessel and the numerous small lateral branch vessels, and to some extent 

 also on the ventral vessel and dorsal to it on the mesentery, the peritoneum is 

 formed of tall narrow cells stuffed with excretory granules (Figs. 61, 68, 69, 

 (Ivanov, 1958a, 1960a). 



