92 



PART I. GENERAL ACCOUNT 



sac is continuous with the membrane of the mesentery and with the intima 

 of the blood vessel. Ventrally the partition between the sacs preserves the 

 full nature of a mesentery, with peritoneal cells amongst which may be seen a 

 number of groups of excretory cells and of cytoplasmic cell bodies of the 

 myocytes of the dorsal vessel. Externally the sacs are clothed with a very thin 

 layer of circular muscle fibres and peritoneal cells (Fig. 61). 



Fig. 61. Transverse section of a male Lamellisabella zachsi at the level of the front end of the 

 sperm sacs. 



axn - longitudinal ridge, rich in nuclei, in wall of spermduct; сое III - metacoele; cv -corpus 

 cardiacum; ep - epidermis; eps - ciliated epithelium of sperm sac; mes - mesentery; ml- longi- 

 tudinal muscle layer; nd - dorsal nerve trunk; per - peritoneum; sp - sperm; sp 1 , sp' 2 , sp 3 - succes- 

 sive stages in spermatogenesis; spd 1 - spermduct; spl -excretory cell of the splanchnopleura; 

 ss - sperm sac; vc - commissural vessel; vd - dorsal vessel; w - ventral vessel. (After Ivanov, 

 1958a.) 



The ciliated epithelium, however, is well developed only in the anterior 

 part of the testis, where it is formed of cylindrical cells. A little farther back 

 in each sac only narrow longitudinal ciliated ribbons lemain, located on the 

 latero-ventral walls; the rest of the epithelium has lost its cilia and comes to 



